exceptional two wire, direct current early type d direct current watthour electromechanical induction meter containing a mercury-motor meter with cast iron mercury chambers and aluminum armature disk. retains original round black back-connected glass "switchboard" case with beveled faceplate and acid-etched "generator no 1" print. the plate containing the dials is made of copper with original white porcelain coating. this electromechanical induction meter operates by counting the revolutions of the aluminum disc which is made to rotate at a speed proportional to the power. the number of revolutions is thus proportional to the energy usage. the metallic disc is acted upon by two coils; one coil produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. a permanent magnet exerts an opposing force proportional to the speed of rotation of the disc - this acts as a brake which causes the disc to stop spinning when power stops being drawn. manufactured by sangamo electric co., springfield, il. the meter dates to 1907-13. the glass encased watthour meter removed from a early 20th century chicago office building. Priced individually