The needle of the potential indicator is mounted at the lower end of
a steel axle, to the upper end of which is fastened a light aluminum
pointer, whereby the deflection of the needle can be read off on a scale
divided directly into volts. The scale is placed within a circular dial
plate with glass cover, giving sufficient room for the pointer to swing
all round, and the needle is placed within a central tube fitting it
closely, which acts as a damper and so makes the instrument almost
dead beat. Tube and dial are in one casting. The electro magnet is of
horseshoe form fastened to a central tubular stand, which also serves
to support the two deflecting coils, one on either side of it. The tube
within which the magnetic needle swings is inserted into the stand,
which is bored out to the external diameter of the tube. The electro
magnet and deflecting coils are wound with from 50 to 100 ohms of fine
insulated copper wire, and an additional resistance coil of from 450
to 900 ohms of German silver is added, which can, however, be short
circuited by depressing a key when the instrument has to be used for
reading low electromotive forces.
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