178 ELECTRICAL MEASUREMENTS 



need be known with exactness. A balance is effected in the usual 

 way. Then by the law of the bridge, 



A - B = RD,. 



The known coil is then shunted by a resistance C which is of 

 such a value that the balance points fall near the ends of the slide 

 wire. This shunted coil is then compared with the coil A which 

 gives ^ 



A - g-pg 

 therefore 



When comparing coils of moderate resistance it may happen 

 that their difference is so great that the slide wire is not of 

 sufficient length; in such a case the larger resistance may be 

 shunted, a comparison effected in the usual way and the shunt 

 allowed for. The accuracy with which the shunt must be known 

 depends on its ratio to the resistance of the coil around which it is 

 placed. 



Calibration of a Slide Wire. In dealing with the slide-wire 

 bridge, it has been assumed that the wire is of uniform resistance 

 per unit length. In order that troublesome corrections may be 

 avoided every effort should be made to have this assumption 

 strictly true. If a case arises where the wire must be tested for 

 uniformity, it may be divided into sections of equal resistance by 

 one of several different methods. If the wire be uniform these 

 sections will be of equal length; if they differ in length, corrections 

 may be determined by which a reading on the wire may be 

 reduced to the value it would have had if the wire had been of 

 uniform resistance per unit length. 



Carey Foster Method of Calibrating a Slide Wire. Referring 

 to the demonstration for the Carey Foster method of comparing 

 two coils (page 176), it is seen that their difference is independent 

 of S and S'; the relative values of S and S', however, determine 

 the position of the balance points on the slide wire. Let A and 

 B be two nearly equal resistances one of which is shunted by a 

 resistance, C, so that I* l\ has the desired value. The two 

 sections of a second slide wire replace the coils S and S' ordi- 



