The missing element is the use of electromagnets in place of permanent magnets. Faraday had shown that the electromagnet produced a much larger magnetic field than the permanent magnet. A chicken and egg problem exists: how can the permanent magnet be energized without the use of a separate generator?
The problem was solved by a number of physicists and inventors in the years 1866-67. The original discovery was probably that of Henry Wilde in Manchester, England in 1866. He found that a small, residual magnetism in the electromagnet core was enough to make the generator magnet self-energizing. The effect was discovered later in the year by Moses Farmer of Salem, Massachusetts, and C. and S. A. Varley of London. At the same time, Werner Siemens of Berlin discovered the effect, and Sir Charles Wheatstone announced the principle to the Royal Society in 1867.
The hand crank on the Transylvania University generator at
the left below shows that it was used for lecture demonstrations.
The larger Kenyon generator is probably the one which was used to power the physics laboratories in the last years of the 19th century. Its considerable weight is invested in the magnetic circuit of the electromagnets; I can say, from personal experience, that it is just barely movable by a senior professor of physics.
Two more examples of generators are shown below. The Denison
generator at the left has a ring armature, but its best point is its wonderful
crank, complete with arrows to indicate the direction of rotation. The VMI
machine at the right is by Ziegler Electric Company of Boston, and is typical
of many small demonstration generators in use at the beginning of the 20th
century. It can produce AC or pulsating DC by using different sets of slip
rings on its commutator.
The generators below are listed in the 1929 Central Scientific Company catalogue as the "Miller-Cowan Dynamo Electric Machine, for study of both alternating and direct currents. This excellent lecture table piece effectively serves to demonstrate: Alternating Currents, Direct Currents, Use of Commutator, Effect of Speed on Rotation, Dynamos -- separately and Self-Excited, Alternators, Direct Current Generators, Dynamos and Motors -- Series -- Shunt -- Compound ... $19.50"
The apparatus at the left is at the United States Military
Academy at West Point, New York, and at the right is a similar apparatus at
the University of Arkansas in Fayetteville. Note that the brushes contact
different points on the commutator shaft in the two instruments.
|This somewhat newer Central Scientific Company generator is at the University of Cincinnati.|
| The Gilley Gramme Machine is listed in the 1916 catalogue
of the L.E. Knott Apparatus Company of Boston at $7.75. This example was in
storage at Williston Northampton School in Easthampton, Massachusetts when
it was photographed by my student, Daniel Hayden, in 1996.
The magnetic field is set up by the electromagnet at the back of the apparatus that is magnetically linked to the two poles surrounding the rotating armature. Projecting from below the armature is the commutator. A glass or light cardboard sheet can be placed on top of the flat apparatus, and iron filings spread on top of the sheet trace out the magnetic field.
This generator, in the collection of Westminster College in Western Pennyslvania, probably dates from the nineteen twenties or thirties. The double set of pullies makes it possible to spin it up to a high speed -- with no load.