  We describe a new type of magnetic field sensor which is termed an
Electro-Mechanical Resonant Sensor (EMRS). The key part of this sensor is a
small conductive elastic element with low damping rate and therefore a high Q
fundamental mode of frequency $f_1$. An AC current is driven through the
elastic element which, in the presence of a magnetic field, causes an AC force
on the element. When the frequency of the AC current matches the resonant
frequency of the element, maximum vibration of the element occurs and this can
be measured precisely by optical means. We have built and tested a model sensor
of this type using for the elastic element a length of copper wire of diameter
0.030 mm formed into a loop shape. The wire motion was measured using a light
emitting diode photo-transistor assembly. This sensor demonstrated a
sensitivity better than 0.001G for an applied magnetic field of $ \sim 1$G and
a good selectivity for the magnetic field direction. The sensitivity can be
easily improved by a factor of $\sim 10 - 100$ by more sensitive measurement of
the elastic element motion and by having the element in vacuum to reduce the
drag force.
