Measurement of Unknown Pressure.--The potentiometer (fig. 610), as ordinarily used, gives direct readings for voltages up to and including 1.6 volts. For pressures higher than 1.6 volts, a volt box or multiplier should be used. After obtaining the standard cell balance, as previously described, place the double throw switch in the position marked E.M.F. The balance for the unknown E.M.F. is obtained by manipulating the tenths switch and rotating the contact on the extended potentiometer wire. The final position of the two contacts in conjunction with the position of the plug at the left of the instrument indicates the voltage under test. As directed above, use key R_{1} for rough adjustment, R_{2} for intermediate adjustment, and key R_{0} for final adjustment. Plug at 1 gives readings for voltage directly from settings of tenths switch and extended wire contact. Plug at .1 shunts the potentiometer circuit so that the voltage measured is .1 of the reading taken directly from the scale. Hence, the readings taken from the setting of the tenths switch and the slide wire contact must be divided by 10. To Balance Galvanometer for Unknown Voltage.--Place plug in hole 1 (fig. 610) for voltages up to 1.6, and in hole .1 for voltages up to .16. Rotate the tenths switch until a condition of balance is obtained exactly or approximately. To secure an exact balance, rotate the contact on the extended wire. The unknown voltage can now be read directly from the position of the tenths switch and the extended wire contact if plug be at 1, or by dividing by 10 if plug be at .1. EXAMPLE.--A balance was obtained with the tenths switch at 1.3, the extended wire contact at 176 and the plug at 1. The voltage under test, therefore, is 1.3176. If the plug at .1 had been used, the same reading would have indicated .13176. To ascertain if the current in the potentiometer circuit has altered during a measurement, it is only necessary to plug in at 1, place the double throw switch on STD. CELL and close the galvanometer key. No deflection indicates that the current has not changed. If the galvanometer deflect, the regulating rheostat must again be adjusted until the galvanometer shows no deflection. To Measure Voltages from 1.6 to 16.--Pressures up to 16 volts may be measured by using a greater voltage across the BA posts (fig. 615). For this purpose a battery of about 20 volts should be used. Insert the large plug at .1 and throw the switch to STD. CELL, then balance the galvanometer by means of the regulating rheostat. When the rheostat has been set to secure a balance, insert the large plug at 1, set the switch on E.M.F. and read the voltage in the usual manner. Multiply the reading by 10.