MEASUREMENT OF POTENTIAL DIFFERENCE 273 



process is then to set c' so that the drop between the terminals of. 

 the standard-cell circuit due to the predetermined potentiometer 

 current will be equal to the e.m.f. of the particular standard cell 

 used. The proper value of I a b is obtained by manipulating 

 the rheostat, Rh. With the switch S thrown to the right, the 

 correct adjustment is shown by the galvanometer remaining in 

 balance. After this, P.D. is measured as before by adjusting c, 

 the switch being thrown to the left. To test the value of I a b, 

 it is necessary to insert the galvanometer in the standard-cell 

 circuit by the double-throw switch. This test is necessary be- 

 cause the battery may not be constant. 



The usual range of a potentiometer is from to about 1.5 volts; 

 for higher voltages it is necessary to use it in conjunction with a 

 volt box. This consists of a high resistance which is connected 

 across the potential difference to be measured. It is divided by 

 taps so that the potentiometer measures a definite fraction, 

 Mo, Moo, M,ooo of P.D. 



Practical Arrangement of the Potentiometer. In order that 

 a potentiometer may attain its highest usefulness, it must be so 

 arranged that the value of P.D. may be read directly from the 

 position of the slider c, no calculation being necessary. That 

 this is possible is seen from the fact that whenever the instrument 

 is used, the current I a b has a definite value and therefore the drop 

 from b to any point on the wire ab is always the same. Conse- 

 quently the scale from which the position of c is read may be so 

 graduated that it gives the drop in volts between b and the 

 various positions of c directly. 



Much ingenuity has been expended in arranging the resistance 

 ab so that while it is brought into a small compass it is accessible 

 at practically all points. The simplest method of doing this is 

 shown in Fig. 156. Fifteen equal-resistance coils are used in 

 series with a slide wire whose resistance is slightly greater than 

 that of a single coil. The scale of the slide wire is divided into 

 1,100 equal parts and the resistance of the whole affair may be 

 about 75 ohms. 



The slide \\irc, represented by DB, is wound in a screw-thread 

 on a marble cylinder 6 in. in diameter; it consists of 11 turns 

 with a total resistance of 5.5 ohms, and is protected from dirt and 

 mechanical injury by a movable hood mounted on a screw thread 



18 



