[Illustration: Fig. 616.--Measurement of current with potentiometer. This is done by measuring the drop in volts across a known low resistance. In the figure S is the standard resistance, and on it are the pressure terminals pp, and the current terminals CC. The potentiometer is connected to the shunt through the posts marked P. The resistance between the points pp is adjusted to an even fraction of an ohm. These resistances are so chosen that in order to determine the current passing through the shunt, after having obtained a potentiometer balance it is only necessary to multiply the potentiometer reading by a simple factor. For instance, in using a .01 ohm standard. It is only necessary to multiply the potentiometer reading by 100, which gives the current reading in amperes; similarly, a .1 ohm requires multiplication by 10, and a .001 ohm by 1,000.] Care of Potentiometer.--The slide wire, although protected to a great extent by the hood, in time accumulates dust and dirt with a thin film of oxide. This will tend to increase the resistance in this part of the circuit owing to poor contact. This wire should, therefore, be cleaned occasionally. To do this, unscrew the stop against which the hood strikes when turned to read zero; then remove the hood and rub the entire slide wire vigorously with a soft cloth dipped in vaseline. _Do not use emery or sand paper as this will destroy the uniformity of the slide wire._ Clean also the steel contact which rubs on the wire, as this becomes glazed after much use. When the potentiometer is not in use, the hood should be screwed all the way down, and the lid put in place to exclude dust. If it be used in a chemical factory, laboratory, or any place where acid fumes are prevalent, this latter precaution is important, because the fumes may attack the slide wire. It is also well to keep the contact surfaces of the switch studs clean and bright by wiping them occasionally with a soft cloth dipped in vaseline. [Illustration: _Res. of leads on each end is equal to 10 divisions of slide wire. The slide wire is divided into 1000 parts--20 for the leads, or 980 divisions. Calibrated scale on Galvanometer:_