176 ELECTRICAL MEASUREMENTS 



The coils to be compared are inserted at a and b (Fig. 98). 

 Let their resistances be A and B', two approximately equal re- 

 sistances S and S' are inserted at c and d; this insures that the 

 balance point will fall near the middle of the slide wire. Let R 

 be the resistance per unit length of the slide wire, L the total 

 length of slide wire in divisions, l\ and h the readings of the slider 

 at balance. Then 



*L A 



S' " B + 



S f " A + n*R + (L - 

 S A + niR + 



+ S' " A + B + mR + n 2 # + 



1 2 R 



S + iS' ~ A + B + mfl + mR + 

 /. A - = JKZ, - Zi 



If A and B are interchanged, 



S B + n l R+ 1 2 R 



So 

 and 



D is the difference of the two readings of the slider. 



The difference of the resistances of the coils is seen to be 

 equal to the resistance of the portion of the slide wire between 

 the two balance points. It is to be noticed that this result is 

 independent of S and S' and of HI and n z , as well as of con- 

 tact resistances, if these factors remain constant during the test. 

 Some convenient device must be adopted for interchanging the 

 coils without removing them from their cooling baths and with- 

 out handling, and as it is the difference of two nearly equal 

 quantities which is involved, extraneous resistances due to change 

 of connections and contacts must be carefully avoided. Fig. 99 

 shows a form of bridge especially designed for carrying out the 

 Carey Foster test. 



The interchange of the coils is effected by raising the contact 

 switch K, turning it through one-half a revolution and then 

 lowering it. 



To adapt the device to the comparison of high as well as low 

 resistances, several pairs of coils (SS f ) must be provided in order 

 that a sensitive bridge arrangement may be maintained. A 

 number of slide wires of different resistance per unit length, 



