MEASUREMENT OF POTENTIAL DIFFERENCE 271 



it through a small coil which might be overheated, it is well 

 to unplug large resistances (1,000 ohms) in Ri and Rz before 

 making the final connections. 



The key should not be left depressed, but released as soon 

 as the galvanometer needle begins to move. The direction of the 

 motion of the needle must be noted and another trial made with 

 a different resistance in R\. The direction of the deflection 

 depends on whether Ri is too large or too small. 



After repeated trials such a resistance will be found that the 

 galvanometer will not deflect. 

 Then 



7? T? ^ l t?_ 2 



& 1 = ^2 5 



K\ 



To make the final balancing, either Ri or R 2 may be ad- 

 justed. One must be sure that all plugs are firmly inserted and 

 that all connections are perfect. The key is to be depressed for 

 as short a time as possible. 



If in any particular case the standard cell is higher in e.m.f. 

 than the cell to be compared with it, the procedure must be 

 changed, for standard cells cannot be used in closed circuits. 

 In this case, an auxiliary battery having a higher e.m.f. than 

 either E\ or E z is used in position E\. The cells are compared 

 with it in succession, or else the circuit is so arranged that 

 both cells can be compared with the auxiliary battery at the 

 same time. The latter is a very accurate method, for it obviates 

 difficulties arising from variations of the current through Ri 

 and R 2 . 



The Potentiometer. In the section on " Calibration of In- 

 struments" some methods of employing standard cells are dis- 

 cussed, the apparatus for which may be assembled in any well- 

 appointed laboratory for electrical measurements. In general, 

 it is much more convenient to use for the purpose pieces of 

 commercial apparatus called potentiometers. These instruments 

 are all more or less convenient arrangements for projecting 

 potentials, so designed as to be direct-reading for voltages up 

 to about 1.5 volts. The principle involved is illustrated by Fig. 

 154. 



ab is a definite resistance along which 'the slider c can be dis- 

 placed. Rh is a rheostat by which the current in ab can be 



