Although electricity contains many parallels with motion, it involves a
fundamentally different force. Electricity is the study of the motion and
effect of charges just as mechanics is the study of the motion and effect
of masses. The subject of electricity was placed on a firm quantitative
basis by the Scottish mathematician and physicist, James Clerk Maxwell
(1831-1879), whose four equations encompass all electromagnetic phenomena.
When Einstein, a generation later, upset almost all of "classical physics,"
Maxwell's equations survived untouched. Electrical demonstrations, especially
those involving very high voltages, constitute some of the most spectacular
in the whole of physics.
Electrical Safety Considerations
The use of electricity in demonstrations entails special hazards from electrical
burns and electrocution. The hazards of electricity (see Table 4.1)
depend on the amount of electrical current, its frequency, its duration,
its path through the body, and the physical condition of the person. Alternating
current at 60 Hz is slightly more dangerous than direct current, but high
frequency currents (greater than a few kHz) are safer because they tend
to flow on the surface of the skin and away from the heart and lungs. The
resistance of the body varies from about 300 ohms to about 100,000 ohms,
and thus even very low voltages can produce lethal shocks. Fatalities have
occurred at voltages as low as 24 volts. For currents that exceed the "let-go"
current (10-20 mA) the person becomes frozen to the circuit, and the
current typically rises to a level of about 25 mA where muscular contractions
onset. Then the person is either thrown clear of the circuit or the current
continues to rise until ventricular fibrillation or cardiac arrest occurs
(at 50-200 mA), assuming the path of the current is through the heart.
Pulsed currents such as one might encounter with the discharge of a
Van de Graaff generator or other charged capacitance present special considerations.
One can endure currents that would otherwise be lethal if the duration
is short enough. For pulses of less than a few seconds duration, the relevant
quantity is the square of the current integrated over the time of the pulse.
Values of I2t greater than about 0.01 A2- sec can
cause electrocution for a typical adult. For a reasonably low body resistance
of 2000 ohms, this translates into an energy of about 10 joules. Severe
shocks can occur at levels 10-100 times lower and can startle one into
an accident, since it is natural to jerk away from such a shock.
The usual safety precautions entail some combination of the following:
copious and redundant insulation, grounding of all exposed metal parts,
interlocks, isolation transformers and ground fault interrupters. Good
practice entails standing on an insulated surface and keeping one hand
in a pocket or behind one's back while working around high voltage. In
performing electrical demonstrations, one should ideally have a knowledgeable
assistant trained in cardiopulmonary resuscitation (CPR) always present.
The probability of resuscitating someone is good if CPR is begun within
three minutes but becomes poor after about six minutes. CPR classes are
offered by many groups such as the American Red Cross and the American
1. E. A. Lacy, Handbook of Electronic Safety Procedures, Prentice-
Hall: Englewood Cliffs, New Jersey (1977).
2. C. F. Dalziel and W. R. Lee, IEEE Spectrum Feb, 44 (1969).
Average Effects of Continuous ac or dc Electrical Currents on Healthy Adults
Electrical Current Biological Effect
1 mA threshold for feeling
10-20 mA voluntary let-go of circuit impossible
25 mA onset of muscular contractions
50-200 mA ventricular fibrillation or cardiac arrest
An electrical discharge is illustrated by connecting a high voltage power
supply to a pair of conducting bars close together at the bottom and farther
apart at the top, between which a discharge strikes at the bottom and rises
to the top and then restrikes at the bottom.
power supply, preferably >10 kV and >1 mA, current-limited
2 bars at least a meter long with insulated mounting base
A device with little practical use but which often appears as a prop in
old science fiction movies is the Jacob's ladder,* named
after the biblical character who had a dream of a ladder to heaven with
angels ascending and descending on it.** It consists of two
vertical conducting bars close together at the bottom and far apart at
the top, like a pair of television rabbit-ear antennas. A high voltage
is applied between the bars causing an electric arc to form at the bottom
where the bars are closest together. The arc then slowly propagates up
the bars until finally it extinguishes, whereupon it instantly restrikes
at the bottom. The Jacob's ladder is sometimes called a "climbing arc."
*Available from Carolina Biological Supply Company, Edmund
Scientific and Frey Scientific Company
To make a good Jacob's ladder, a voltage in excess of 10 kV and a current
in excess of a few milliamperes is required. The voltage primarily determines
the length of the arc, and the current determines its intensity. The device
can be run with either ac or dc current. The voltage source must be such
that it can be short-circuited indefinitely without harm. Transformers
designed for neon signs are good because they have a large leakage inductance
between their primary and secondary windings so as to limit the short-circuit
current. With a normal transformer, one can place a suitable impedance
(inductance preferred, but resistance or capacitance will do) so as to
limit the output current to the rated value. For example, if one has a
transformer with an output rated at 20 kV and 10 mA and if the input is
115 V, the desired impedance is 1152/.01/20000 = 66 ohms. This
could be either a resistor with a power rating of >1152/66 =
200 watts, an inductor of 66/2(pi)f = 175 millihenries or a capacitor of
1/66×2(pi)f = 40 microfarads. The inductor must have enough iron
so as not to saturate and a reasonable Q at f = 60 Hz (much less than 66
ohms of winding resistance). The capacitor must have a voltage rating greater
than 115×2½ and not be electrolytic but must be
designed for ac service. Motor starting capacitors are suitable.
When the device is turned on, the full voltage initially appears across
the bars causing an arc to form at the closest point, where the electric
field is highest. The voltage then drops to a low value because the impedance
of the arc is low. The arc heats the air, and the hot air begins to rise
because it is less dense than the surrounding air. As the arc rises, its
impedance increases, and the voltage between the bars rises. Eventually
the arc gets so long that the voltage is not sufficient to sustain it,
and the arc goes out. Then the voltage rises to its full open-circuit value,
and the arc restrikes at the bottom. There is also a magnetic force causing
the arc to rise, but the dominant effect is the heating of the air as can
be demonstrated by tilting the whole device on its side.
The Jacob's ladder illustrates electrical discharges and the variation
of the density of air with temperature. The principle is used in the horn
gap on power transmission lines and in transformer yards for dissipating
the arc of disconnects. It could serve as an introduction to a discussion
of the physics of plasmas (ionized gases) and electrical breakdown. If
the Jacob's ladder is operated inside a closed tube, the tube fills with
a red-brown oxide of nitrogen. The oxide may then be aspirated through
water to form nitric acid, illustrating the fixation of atmospheric nitrogen
by lightning. The Jacob's ladder also converts oxygen gas (O2)
into ozone (O3), a pungent, highly reactive allotrope of oxygen.
This demonstration is potentially very dangerous. If currents of more than
a few milliamperes are used, the Jacob's ladder will severely shock anyone
who comes too close to the bars while it is in operation. Some measure
of safety is provided if the secondary is not grounded so that one would
have to be close to both bars simultaneously to receive a shock. The safest
arrangement would be with the whole apparatus enclosed in a transparent
case which can also serve to support the bars at the top. The case also
acts as a resonant acoustical cavity, enhancing the sound of the arc.
1. H. Strand, Popular Science 184, 110 (1964).
Van de Graaff Generator
A Van de Graaff generator is used to illustrate many principles of high
voltage electrostatics, such as making a person's hair stand on end.
*Available from Central Scientific Company, Edmund Scientific,
Fisher Scientific, Frey Scientific, Klinger Educational Products Corporation
and Sargent-Welch Scientific Company
Van de Graaff generator*
grounded conducting sphere
grounded pointed needle
hand mirror or Polaroid camera (optional)
A standard physics demonstration is the Van de Graaff generator named after
the American, Robert J. Van de Graaff (1901-1967), who developed it in
the 1930's. A Van de Graaff generator can be made with a hollow metal dome,
a belt to transfer charge to the dome and a motor, preferably with variable
speed control[1-5]. More commonly, Van de Graaff generators are purchased
from vendors of scientific demonstration apparatus. The limiting voltage
is determined by the electrical breakdown of air (about 30 kV per centimeter
at atmospheric pressure), and so a Van de Graaff generator with a 10-cm-radius
dome is capable of producing up to about 300 kV. The voltage may be considerably
less if the humidity is high or if sharply pointed, grounded conductors
are nearby to allow corona to discharge the dome. The dome and insulated
supporting column should be carefully cleaned before use. Despite common
belief, the electrical breakdown of air is only slightly affected by humidity,
but surface leakage is greatly affected by moisture.
The standard demonstration involves having a volunteer stand on an insulated
stool (of a height greater than the diameter of the dome to ensure adequate
insulation) and place a hand on the dome while the Van de Graaff generator
is turned on. After a few minutes, the volunteer's hair will begin to stand
on end. A hand mirror is useful to allow the person to see what is happening
(but don't hand it to someone charged to high voltage!). A Polaroid photograph
also makes a good souvenir.
It is best to choose someone with medium length, fine, dry hair. Shaking
the head helps make the hair stand up. The person should be well away from
any grounded conductors and told to leave the hand on the dome at all times
until told to remove it. When finished, the dome should be slowly discharged
by bringing a grounded, pointed needle near the dome while the person is
still in contact with it. The hair will drop back into place as corona
from the needle discharges the dome, and the volunteer can then step down
from the stool. The stool should be cleaned before use. If the stool is
at all tipsy, the person should be given help up and down.
Afterwards, one could show the distance over which a spark can be made
to jump by bringing a grounded sphere near the dome. The increased capacitance
of the sphere makes the spark more intense and visible. If the grounded
sphere is mounted on a flexible rod, the attractive force due to the induced
charge in the sphere will make the support rod bend toward the dome. When
a spark occurs, neutralizing the charge on the grounded sphere, the support
rod will straighten, producing a mechanical vibration.
Other demonstrations can also be done with the Van de Graaff generator.
Puffed rice placed on the dome will fly off in all directions when the
generator is turned on. A pinwheel placed on the dome can be made to rotate
from the corona given off the points. A fluffy cotton ball or silvered
balloon[7-9] released near the generator will be attracted to the dome
and then repelled after it touches the dome and becomes charged with the
opposite polarity. Soap bubbles can be blown toward the dome by someone
at ground potential or away from the dome by someone touching the dome[10,11].
Bits of fur can be tossed into the region between a charged and grounded
dome and illuminated with a strong light to illustrate qualitatively the
shape of the electric field.
The Van de Graaff generator demonstrates a number of principles foremost
of which is the relationship between charge and voltage, Q = CV, where
C is the capacitance of the dome. The capacitance of a sphere with respect
to infinity is about 1.1 pF per centimeter of radius. The electrical energy
stored in the Van de Graaff generator is CV2/2. Energies below
about 10 joules (corresponding to a dome with a 27-cm radius) are relatively
safe. Hair stands up because it acquires charges of like sign which tend
to repel one another. The Van de Graaff generator was one of the earliest
particle accelerators used for nuclear physics research. Modern Van de
Graaff generators and other similar electrostatic accelerators are still
used for that purpose and are capable of voltages in excess of 20 million
Small Van de Graaff generators (less than a few hundred thousand volts)
are relatively safe since the charging current is typically only a few
microamperes and the capacitance is too small to allow much of a charge
buildup, provided additional capacitance is not added externally. Even
so, it is probably best not to let someone with a weak heart touch the
dome. The greater danger is that a spark to the person will be startling
and might prompt a recoil that could cause injury.
1. R. J. Van de Graaff, et. al., Rep. Progr. Phys. 11, 1 (1946).
2. A. D. Moore, ed., Electrostatics and its Applications, John
Wiley & Sons: New York (1973).
3. H. Walton, Popular Science 185, 142 (1964).
4. E. Francis, Popular Electronics 24, 57 (1966).
5. C. L. Strong, Scientific American 225, 106 (Aug 1971).
6. L. B. Loeb, Electrical Coronas, University of California Press:
Berkeley, CA (1965).
7. P. Hood, Am. Journ. Phys. 14, 445 (1946).
8. A. V. Baez, Am. Journ. Phys. 25, 301 (1957).
9. M. D. Daybell and R. J. Liefeld, Am. Journ. Phys. 31, 135
10. G. R. Gore, Science Teacher 39, 48 (1972).
11. R. Prigo, Am. Journ. Phys. 44, 606 (1976).
12. J. L. Smith, Phys. Teach. 27, 358 (1989).
13. P. H. Rose and A. B. Wittkower, Scientific American 223,
24 (Aug 1970).
14. A. D. Moore, Electrostatics: Exploring, Controlling & Using
Static Electricity, Doubleday: New York (1968).
A Tesla coil, because of its high frequency, provides a safe way to demonstrate
very-high-voltage, high-current phenomena.
*Available from Carolina Biological Supply Company, Edmund Scientific,
Fisher Scientific and Frey Scientific Company
The Tesla coil is a type of resonant transformer that is capable of producing
high frequency voltages of upwards of a million volts. The Tesla coil was
developed by Nikola Tesla (1856-1943), a contemporary and rival of Thomas
Edison (1847-1931). Tesla's biography makes especially interesting reading[1-4].
In 1899 Tesla produced 135 foot long discharges, 200 feet above the earth
with a 12 million volt coil at his Colorado Springs laboratory, and the
overload on the power line set fire to the alternator of the Colorado Springs
Electric Company. Tesla imagined using his invention not only for wireless
communications around the world but also for the widespread dissemination
of electric power without the use of wires.
Because of its high frequency, the Tesla coil provides a relatively
safe way to demonstrate very high voltage phenomena. A large Tesla coil
is probably the most spectacular of all electrical demonstrations. The
Tesla coil also makes an excellent student project since there is much
room to experiment and to optimize its performance. The followers of Tesla
almost constitute a cult. There is an International Tesla Society**
and a Tesla Coil Builders Association*** that publishes a journal
containing much useful information. Old issues of electronic hobby magazines[5-10],
scientific journals[11-14] and books[15-17] have construction details.
A recent book that contains much practical design information is Modern
Resonance Transformer Design Theory, by D. C. Cox.****
**International Tesla Society, 330-A West Uintah Street,
Suite 215, Colorado Springs, CO 80905-1095 (303) 570-0875
***Tesla Coil Builder's Association, 3 Amy Lane, Queensbury,
NY 12804 (518) 792-1003
****Available from Resonance Research, Corp. R1, Shadylane
Rd., Box 320A, Baraboo, WI 53913 (608) 356-3647. This company also produces
custom-made Tesla coils and other electrical demonstration apparatus.
There are many other possible uses for the Tesla coil aside from producing
long and impressive sparks in the air surrounding the top of the coil.
It should be possible to illuminate a fluorescent or gas-filled tube held
in the hand some distance away. A hydrogen-filled balloon on the end of
an insulated rod can be made to explode when brought near the top of the
coil. It is possible to draw the current through one's body by bringing
a long metal rod held in the hand near the top of the coil. With a large
coil one should be able to light an incandescent bulb in series with the
rod. One should use the highest voltage bulb possible to get the most light
for a given current. If a sufficiently well-insulated platform is available,
one should be able to stand with bare feet on a metal plate (or more impressively
sit in a washtub full of water) connected by a wire to the top of the coil
and light a discharge tube or clear incandescent light bulb held in the
hands or draw sparks off the tips of the fingers. In doing this demonstration,
the hair should be wetted and metal thimbles worn on the fingers to prevent
The Tesla coil is essentially a radio transmitter without the antenna,
and thus Tesla rightly deserves some of the credit for the invention of
the radio, although his interest was more in the transmission of electric
power than in communication. It is capable of producing severe radio interference
and thus should be operated inside a shielded cage and only for brief intervals.
A suitable detector, such as a large loop of wire with a pair of conducting
balls on the ends between which sparks can jump, placed some distance from
the coil, demonstrates the propagation of these radio waves. One can also
point out that the discharge is a good example of a fractal.
The Tesla coil
consists of primary and secondary resonant circuits tuned to the same frequency
(f = 1/2(pi)(LC)½) of the order of 100 kHz to 1 MHz and
designed to have the highest possible Q (lowest circuit loss). The secondary
capacitance Cs is predominantly distributed capacitance due
to the proximity of adjacent windings, and thus the secondary behaves much
like an open circuit transmission line with a resonance whenever the electrical
length is an odd multiple of a quarter wavelength. The Tesla coil is normally
designed to operate at the lowest resonant frequency of the secondary to
avoid a voltage maximum other that at the top of the secondary coil. If
the coupling between the primary and secondary is adjusted to be near the
critical value (kQ ~ 1), then the secondary voltage is given approximately
by Vs = Vp(Ls/Lp)½
independent of the turns ratio. In practice one would normally construct
the secondary coil and then adjust Cp, Lp and the
coupling constant k to give the largest Vs for a given Vp.
The Tesla coil will have to be retuned depending upon what is connected
to the secondary. Taps on the primary are useful for this purpose.
The input voltage Vp can be produced in a number of ways.
The usual way is to connect the input through a rotary spark gap to the
secondary of a high voltage (several kV) transformer connected to the ac
power line through a current-limiting inductor. The opening and closing
of the rotary gap (typically a few hundred pulses per second) creates a
ringing voltage waveform after each opening. The spark gap and transformer
can also be inserted in series between Cp and Lp
to create a ringing waveform after each closing of the switch. The acoustically
noisy spark gap can be replaced with a thyratron or ignitron and triggered
from a source that is phase-locked to the 60 Hz power line. The voltage
Vp can also be derived from a high voltage, vacuum tube oscillator
*****See an old issue of The Radio Amateur's Handbook,
published yearly by the American Radio Relay League: 225 Main Street, Newington,
With a Tesla coil, it is possible to touch the very high voltage secondary
without being electrocuted because of the skin effect. The high frequency
currents flow on the surface of the skin rather than through the body where
damage to the heart could occur. Furthermore, the muscles of the body are
less sensitive to high frequencies than they are to dc or low-frequency
ac. Even so, it is probably best not to allow anyone with a weak heart
to come into contact with the high voltage. In fact, with large Tesla coils,
the use of volunteers is not recommended. In some Tesla coil designs the
primary has 60 Hz voltages present, and these could be lethal. A more serious
danger is the possibility of burns to the skin, especially if the discharge
strikes the skin directly. For this reason, the discharge should only be
drawn to the body through a metal object which makes a large area of contact
with the skin. If the discharge strikes an unsuspecting person, the result
would surely be startling, and the recoil could cause injury.
If the Tesla coil is used to light a fluorescent tube, care should be
taken to avoid breaking the tube. The powder that lines the inner walls
of such tubes as well as the mercury that they contain are highly poisonous.
1. M. Cheney, Tesla: Man Out of Time, Prentice-Hall: Englewood Cliffs,
New Jersey (1981).
2. I. Hunt and W. W. Draper, Lightning in his Hand: The Life Story
of Nikola Tesla, Sage Books: Denver (1964).
3. T. C. Martin, The Inventions, Researches and Writings of Nikola
Tesla, Omni Publications: Hawthorne, California (1977).
4. J. J. O'Neill, Prodigal Genius: The Life of Nikola Tesla,
Neville Spearman: London (1944).
5. L. Reukema, Experimenter 4, 309 (1925).
6. R. C. Dennison and E. H. McClelland, Electrical Journal 28,
7. K. Richardson, Popular Electronics 12, 72 (1960).
8. C. Caringella, Popular Electronics 21, 29 (1964).
9. E. Kaufman, Popular Electronics 21, 33 (1964).
10. V. Vollano, Popular Electronics 71, 29 (1989).
11. G. Breit and M. A. Tuve, Nature 121, 535 (1928).
12. G. Breit, M. A. Tuve, O. Dahl, Phys. Review 35, 51 (1930).
13. C. R. J. Hoffmann, Review of Scientific Instruments 46, 1
14. I. Boscolo, G. Brautti, R. Coisson, M. Leo, A. Lunches, Review of
Scientific Instruments 46, 1535 (1975).
15. H. S. Norris, Induction Coils - How to Make, Use, and Repair
Them, Spon and Chamberlain (1909).
16. G. F. Haller and E. T. Cunningham, Tesla High Frequency Coil
- Its Construction and Uses, D. Van Nostrand Co.: New York (1910).
17. A. D. Bulman, Models for Experiments in Physics, Crowell
Co.: New York (1966).
A screen cage large enough for a person to enter is used with a Tesla coil
or Van de Graaff generator to illustrate the fact that a closed conducting
surface is an equipotential.
closed screen cage
Tesla coil or Van de Graaff generator
2 fluorescent tubes (optional)
transistor radio (optional)
metallic tinsel (optional)
The Faraday cage is simply a cage constructed of metal or screen so as
to completely enclose the interior with an electrically conducting boundary.
A Faraday cage large enough to put a person in allows even more dramatic
demonstrations. In such a case, a door should be provided to allow the
person to enter and leave. In 1837, Faraday performed an experiment in
which he had someone enclose him with his field-measuring instruments in
a large (12-foot) conducting cube charged to a high voltage until sparks
flew from its corners, and proceeded to convince himself that he could
not detect the fact that the cage was electrically charged. The Faraday
cage is often used in conjunction with a Tesla coil or Van de Graaff generator.
The most spectacular demonstration is to put a person inside a grounded
Faraday cage and turn on a large Tesla coil or Van de Graaff generator
outside the cage. It is especially good to have a celebrity known to the
audience enter the cage. The discharge from the Tesla coil or the sparks
from the Van de Graaff generator strike the cage but do not hurt the person
inside. The person inside can hold a fluorescent tube which will not light
and can even touch the inside of the cage without harm. The electric currents
flow on the outside of the cage, and the interior of the cage is all at
the same electric potential. The person should not stick a finger or other
object through a hole in the cage, however. If someone in the front row
is given a fluorescent tube, it can be made to light while the one in the
cage does not. After the demonstration, one can point out that the person
in the cage was in fact safer than those in the front row.
With a sufficiently well-constructed Faraday cage, it should be possible
to put a portable transistor radio inside and not be able to receive any
radio stations on either the am or fm bands. To do this requires all the
seams to be tightly sealed electrically and the holes in the conductor
not to be too large. This provides a sensitive way to test the quality
of the Faraday cage.
With a sufficiently large Van de Graaff generator or Tesla coil and
an insulated platform for the cage to rest on, it is possible to raise
the voltage of an ungrounded cage to a high level with a person inside.
Metallic tinsel such as used to decorate Christmas trees when attached
to the outside of the cage will stick out like the hair of a person on
a Van de Graaff generator, but similar tinsel inside the cage will show
The Faraday cage illustrates a number of important safety considerations
for someone caught in a lightning storm or required to work with high voltage
electricity. The safest place to be in a lightning storm is inside a closed
metal container such as a building with a steel frame or an automobile
(but not because of the rubber tires, which are far too small to prevent
a lightning strike and contain carbon black, making them relatively poor
insulators). A chicken coop or a garbage can with a lid would also do!
One of the functions of a lightning rod is to shield the interior of a
building by carrying the corona currents from the air around the outside
of the building through a grounded conductor thereby making the building
all at the same electrical potential. High voltage electrical circuits
are normally constructed inside grounded metal enclosures to protect someone
outside the enclosure. The metal enclosure forms an equipotential surface
at the same voltage as the earth and prevents any of the voltages inside
the cage from reaching the outside.
The Faraday cage is named after Michael Faraday (1791-1867), a self-educated,
English physicist and chemist whose lectures for the public in the 1840's
became so popular that they helped save the Royal Institution of Great
Britain from near bankruptcy. His lectures were attended by Charles
Dickens and by Prince Albert, the husband of Queen Victoria, and Prince
Edward, her son (later Edward VII). In 1826 Faraday also began a series
of immensely popular, special Christmas lectures for children which have
continued up to the present. Since 1966, these lectures have been televised
and broadcast throughout Britain.
There are no safety hazards to the person inside the cage so long as the
cage is tightly shut and no part of the body protrudes through holes in
the cage. For the safety of those outside the cage, the cage should be
thoroughly grounded, but this is not required to protect the person inside.
With a large Tesla coil, the experience can be frightening, and so it should
only be done with a brave and willing volunteer.
1. G. Caroe, The Royal Institution: An Informal History, John Murray:
Gas Discharge Tube
A partially evacuated tube filled with various gases at low pressure and
connected to a high voltage electrical source illustrates properties of
gas discharge tube
high voltage power supply, current-limited
various nonflammable gases
pressure, voltage and current meters (optional)
Electrical discharges in low pressure gases make colorful and educational
demonstrations[1-4]. The apparatus required consists of a glass tube a
meter or two long and at least several centimeters in diameter. The tube
can be sealed off at the ends with rubber stoppers. One of the stoppers
contains a hole through which a glass tube can be inserted to allow the
tube to be filled with various gases and partially evacuated. An electrode
of some type such as a nail can be passed through each stopper and connected
to a high-voltage, low-current source of electricity. Direct current is
preferred, but 60 Hz from the power lines or even higher frequency from
a small Tesla coil can also be used. A voltage of 1000 volts or more is
required, but the current can as small as a few milliamperes. Higher currents
produce a brighter discharge but increase the danger of electrocution.
The discharge is most stable if the power supply has a relatively high
output impedance, since the current drawn by the discharge is a sensitive
function of the applied voltage. The tube can be evacuated by means of
a water aspirator, but a mechanical vacuum pump capable of a producing
a vacuum of a few millitorr is preferred. Pressure, voltage and current
meters visible to the audience are useful additions. If a vacuum pump is
not available, some effects can be seen using a pre-sealed discharge tube
such as a fluorescent lamp or other special gaseous discharge lamp such
as a geissler tube*.
*Available from Carolina Biological Supply Company, Central
Scientific Company, Frey Scientific Company and Sargent-Welch Scientific
The tube is initially filled with air at atmospheric pressure, and the
voltage is applied. Normally, there will be no glow from the tube and no
current drawn. As the tube is slowly evacuated, a pressure is reached at
which current abruptly begins to flow, and sparks are seen emanating from
the electrodes. As the pressure is further reduced, a steady violet glow
will appear throughout the tube. At even lower pressure, alternating light
and dark bands (Crookes and Faraday dark spaces) can be seen along the
length of the tube, until eventually the glow is completely extinguished.
Note that the striations are always convex toward the cathode. The pump
can then be turned off, and the progression repeated in reverse as air
is bled back into the tube. The demonstration can then be repeated with
other nonflammable gases such as neon, carbon dioxide and argon. Each will
produce a glow with a distinctive color characteristic of the gas. A magnet
placed near the discharge can be used to effect its shape. It can be pointed
out that a glow discharge tube does not obey Ohm's law.
This demonstration illustrates how the electrical conductivity of a gas
varies with pressure. Normally, gases are electrical insulators because
the molecules of the gas are electrically neutral, consisting of negative
electrons bound to equally positive nuclei. The few free charges that do
exist, perhaps as a result of cosmic rays, are accelerated by the electric
field along the length of the tube but collide with neutral gas molecules
and lose their energy before they are able to ionize them. As the pressure
is reduced, the mean free path for slowing down of the free charges is
lengthened, and the charges acquire enough energy to ionize other gas molecules,
producing additional free electrons, which in turn ionize others, resulting
in electrical breakdown. The alternating light and dark bands result from
electrons emitted from the cathode that are alternately accelerated and
then stopped as they acquire sufficient energy to be absorbed by the gas.
At even lower pressure, the mean free path will exceed the length of the
tube, and the free charges will rarely collide with the gas molecules,
and a discharge will not occur. The light emitted is a result of excitation
of the gas atoms from their ground state to excited states and the subsequent
emission of photons of light as they decay back to their ground state.
Since atoms of different chemical elements have different energy levels,
the color of the discharge depends upon the gas used.
This demonstration can serve as an introduction to the subject of plasma
physics. Plasmas represent the fourth state of matter, along with solids,
liquids and gases. Plasmas represent the end result of raising the temperature
of any substance, most of which transform successively through each of
the four states. Plasmas differ from ordinary gases in that they are electrical
conductors, a difference that is as remarkable as the differences among
the other three states. Plasmas are relatively rare on the earth but constitute
upwards of 99% of the matter in the universe. Examples of plasmas in nature
include the Aurora Borealis, the ionosphere of the earth, the solar wind
and the material of the sun and stars. Laboratory plasmas are of great
current interest because certain atoms such as isotopes of hydrogen at
temperatures in excess of 100 million degrees will release energy by the
process of nuclear fusion, a potentially clean and abundant source of energy[6,7].
The main hazard is electrical shock from the voltage connected to the tube.
There is also a possibility of implosion of the glass tube when it is evacuated
and explosion if it is pressurized above atmospheric pressure (not recommended).
1. F. A. Maxfield and R. R. Benedict, Theory of Gaseous Conduction and
Electronics, McGraw-Hill: New York (1941).
2. C. L. Strong, Scientific American 198, 112 (Feb 1958).
3. B. Z. Shakhashiri, Chemical Demonstrations, The University
of Wisconsin Press: Madison, Wisconsin, Vol 2 (1985).
4. H. F. Meiners, W. Eppenstein, R. A. Oliva and Thomas Shannon, Laboratory
Physics, John Wiley & Sons: New York (1987).
5. F. F. Chen, Introduction to Plasma Physics, Plenum Press:
New York (1984).
6. A. S. Bishop, Project Sherwood, Addison-Wesley Publishing
Company: Reading, Massachusetts (1958).
7. T. K. Fowler and R. F. Post, Scientific American 215, 21 (Dec
Wimshurst Electrostatic Generator
A Wimshurst electrostatic generator is used to produce high voltages at
moderate currents to illustrate many principles of electrostatics.
*Available from Carolina Biological Supply Company, Central
Scientific Company, Edmund Scientific, Fisher Scientific, Frey Scientific
Company and Sargent-Welch Scientific Company
Wimshurst electrostatic generator with accessories*
A simple triboelectric device capable of producing moderately high voltages
for electrostatic demonstrations is the Wimshurst generator, invented in
1878 by James Wimshurst. Other similar devices were invented by the German
physicists, Toepler, Holtz and Musaeus and more recently by the American,
A. D. Moore who performed interesting lecture demonstrations with his Dirod
generator. In addition to making sparks jump through the air, these
devices are capable of operating X-ray tubes, cathode ray tubes, and most
types of vacuum and gaseous discharge tubes. They can be used to deflect
the flame of a candle burning near a pointed electrode at high voltage
or between the plates of a charged capacitor. A dust or ethanol vapor
explosion can be ignited with the spark from these machines. They can be
used to drive an electrostatic motor or a pinwheel with corona or to ring
a set of chimes consisting of a metal ball suspended with an insulated
thread between two bells at opposite electric potential or to separate
a mixture of salt and pepper or to precipitate smoke from the air[3-8].
Accessories for such demonstrations are available from the same vendors
who supply the Wimshurst generators, although many can be constructed from
inexpensive, readily available components.
These devices work by means of rotating plastic or glass plates that become
electrically charged and transfer their charge to a set of external combs
which are typically connected to a pair of Leyden jar capacitors. The disks
can be rotated by means of a hand crank connected to the disks with an
insulated belt. A useful improvement is to add a motor drive with variable
speed control. The capacitors can be discharged through a pair of metal
balls whose proximity to one another can be adjusted. The distance over
which a spark will jump between two spherical balls can be used as a reasonably
accurate measure of the voltage (see Table 4.2). Although these devices
produce lower voltage than Van de Graaff generators, they can produce moderately
large currents. In most cases the Leyden jars can be removed from the circuit
to illustrate the difference between a high-current and a low-current discharge
of similar voltage.
Most commercially available Wimshurst generators are designed so that a
non-lethal amount of electrical energy is stored in the Leyden jars. However,
a discharge to the body is surely painful, and might startle one into an
accident. If significant additional capacitance is connected to the output
of the Wimshurst generator, the hazard can increase substantially. The
spark can also cause burns and ignite flammable or volatile materials nearby.
1. A. D. Moore, Electrostatics: Exploring, Controlling, and Using
Static Electricity, Doubleday-Anchor: Garden City, New York (1968).
2. M. Robinson, Am. Journ. Phys. 30, 366 (1962).
3. M. Robinson, Bibliography of Electrostatic Precipitator Literature,
Southern Research Institute: Birmingham, Alabama (1969).
4. F. T. Cameron, Cottrell, Samaritan of Science, Doubleday:
Garden City, New York (1952).
5. H. J. White, Industrial Electrostatic Precipitation, Addison-Wesley:
Reading, Massachusetts (1963).
6. A. D. Moore, ed., Electrostatics and its Applications, John
Wiley & Sons: New York (1973).
7. J. Bohm, Electrostatic Precipitators, Elsevier Scientific
Publishing Company: New York (1982).
8. J. S. Miller, Physics Fun and Demonstrations, Central Scientific
Company: Chicago (1974).
9. W. R. Mellen, Phys. Teach. 27, 86 (1989).
Approximate Gap Spacing (in cm) For Electrical Breakdown Between two Identical
Spheres of Varying Diameters
Sphere Diameter (cm)
kV 2.5 3.0 4.0 5.0 10.0
10 0.30 0.30 0.30 0.30 0.30
20 0.61 0.61 0.61 0.61 0.61
30 0.95 0.95 0.95 0.95 0.95
40 1.40 1.32 1.30 1.30 1.30
50 2.00 1.82 1.73 1.71 1.65
60 2.81 2.40 2.21 2.16 2.02
70 4.05 3.16 2.80 2.68 2.41
80 4.40 3.50 3.26 2.82
90 4.40 3.93 3.28
100 4.76 3.75
A thin wire or strip of aluminum foil is vaporized by discharging a large
capacitor into it.
*The same apparatus can be used in a can crusher demonstration
(see Chapter 5)
60-µF, 10-kV capacitor with charging circuit*
class A ignitron or spark gap switch
large voltmeter, 10-kV full scale (optional)
Thin copper wire (#22 gauge) or strip of aluminum foil
The operation of a capacitor is explained to the audience. It is likened
to a car battery, but with much higher voltage. The capacitor is charged
to a good fraction of its rated voltage and then discharged with an ignitron
or spark gap switch into a thin wire or strip of aluminum foil, about 20
cm long which is instantly vaporized. The audience should be warned to
cover their ears, since the resulting noise can be quite loud. A protective
piece of plexiglas is placed between the wire and the audience, and the
demonstrator wears protective eyewear and stands at least six feet away.
This demonstration exhibits in a dramatic way the fact that electricity
is just another form of energy. In the process, the electric energy is
converted into five other forms--motion, heat, sound magnetism and light.
The wire or foil usually completely disappears, although it is doubtful
that it all vaporizes. More likely, a portion vaporizes and the remainder
is flung into some distant corner of the room by the magnetic forces.
The stored electrical energy is CV2/2 or about 1920 joules
when the 60-µF capacitor is charged to 8 kV. This can be likened
to a 50-kg person raised 3.9 meters into the air, or the energy in about
8 food calories.
The amount of energy contained in a capacitor of this size is deadly. Electrocution
can result instantly if one comes into contact with the terminals of the
capacitor while it is charged. The wire explodes with considerable force
and noise. Some voltage is generally left on the capacitor after the wire
explodes. One should practice at reduced voltages, and take the necessary
precautions to prevent the hot wire from being thrown out into the audience.
The audience should be warned of the impending noise. Capacitors have been
known to short internally and explode. The capacitor should not be in direct
line-of-sight of the audience. The charging supply should be interlocked
so that a curious spectator can't initiate a charge after the lecture.
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J. C. Sprott