MEASUREMENT OF POTENTIAL DIFFERENCE 275 



B to A, 1.61 volts. One division on the vertical scale, corre- 

 sponding to^one turn, is therefore equivalent to 0.01 volt, and 

 one division on the hood to 0.0001 volt. 



The instrument is designed for use with the commercial Weston 

 cell, which has no appreciable temperature coefficient, so no 

 temperature adjustment is provided. Such cells may, however, 

 differ from one another slightly in e.m.f. (a few ten thousandths 

 of a volt), and each is accompanied by a certificate giving its 

 true voltage. In order that the instrument may be conveniently 

 used with different cells, the rheostat between and A is pro- 

 vided. The standard cell is connected between the movable 

 arm T and the point O r .5. The resistance of each coil of OA is 

 0.005 ohm; so with the standard current of %o amp. flowing, 

 the drop through each is 0.0001 volt. There are 19 of these 

 coils; consequently, cells of e.m.f s. from 1.0170 to 1.0189 volts 

 can be used, the arm T being set to correspond to the particular 

 cell used. 



By means of a double-throw switch the galvanometer may be 

 quickly transferred from the standard-cell circuit to that marked 

 E.M.F; as the two circuits are entirely distinct, no resetting of the 

 instrument is necessary when checking the potentiometer current. 

 This is a very great convenience. 



The process of making a measurement is to set the double- 

 throw switch on the point marked " Standard Cell," and vary the 

 rheostat to get zero deflection; this adjusts the potentiometer 

 current to its standard value. Then throw the switch to 

 "E.M.F." and balance, using the voltage slides without altering 

 the rheostats, and read off V directly in volts. To check the 

 potentiometer current, simply throw the switch to "Standard 

 Cell" and press the key. No resetting is necessary. 



For satisfactory action it is necessary to apply a little vaseline 

 to the slide wire occasionally. 



Low-scale Arrangement. If the current through BA be re- 

 duced to Joo amp., the drop through each coil and through the 

 slide wire becomes Jf oo volt, and the entire range of the potenti- 

 ometer is 0.16 volt. By removing the plug at K from socket 1 

 to socket 0.1 the resistance OB is shunted by S, which is of such 

 a value that one-tenth the total current flows through OB. 



