THE LOCALISATION OF BREAKS AND FAULTS. 389 



tion by 2*34 gives the total deflection which would be produced 

 if the whole of the resistance were in shunt to the galvanometer 

 (i.e., the four sliders being on the extreme right of their respective 

 scales giving a slide reading of 10,000), and this total deflection 

 is, of course, proportional to the total current in the line. It 

 will be evident that as there are 10,000 possible slide readings 

 there are also the same number of possible multiplying powers. 

 The universal shunt, equipped in this manner with vernier 

 scales on the potentiometer principle, comes in useful for many 

 other purposes than a galvanometer shunt. It takes the duty 

 of the Kelvin slides in the various tests for which these are 

 usually employed. 



The four scales are arranged in one box in the form of circu- 

 lar dials (Fig. 231). With it is also included an independent 

 standard resistance of 100,000 ohms for use in taking the con- 

 stant in Schaefer's test, or as the third arm of a bridge of 

 which the slides form the ratio arms, or in series with the 

 shunted galvanometer in bridge tests. The remaining three 

 terminals on the box are marked for using the shunt with 

 galvanometer, as before explained. 



A useful feature in the dial box is that the glass covers for 

 protecting the contacts from dust can be easily detached for 

 cleaning. The coils are of manganin, specially protected against 

 air and moisture, and the contacts are of platinum and gold. 



When resistances, not as high as in insulation tests on 

 cables, are measured by deflection test, the joint resistance 

 of the galvanometer and shunt must be taken into account, 

 unless the deflections to be compared are exactly reproduced. 



In any position (S) of the slider in a universal shunt of 

 R ohms the joint resistance is 



(^±^I^ohms. 

 G + K 



When the slider is at the shunt resistance is nil, and 



therefore the joint resistance is nil. As the slider is moved 



away from the resistance of the shunt increases, and when 



C + R 

 it has reached the value -^ — the joint resistance is at its 



, G + R 

 maximum, namely — -. — . 



