140 



ELECTRICAL MEASUREMENTS 



a b 



m-= L, then V ab will coincide in both magnitude and direction 

 with IR; that is, the shunt acts as if it were non-reactive. 



This method of obtaining balance of inductances, proposed by 

 A. Campbell, 4 is indicated in Fig. 64. 



The shunt resistance, R, is in the form of a straight strip (or 

 tube). The potential leads are copper 

 strips of the same width as the main 

 resistance. They are carried along the 

 body of the shunt to about the middle 



of its length, only a very thin layer of 



TIL " insulating material being interposed, 



FIG. 64. A. Campbell's an( j t h en b ent perpendicularly and at- 

 design for non-inductive J . 



shunt. tached to the potential terminals, a 



and b. By this means, the mutual in- 

 duction may be made practically to balance the self-induction. 

 The balance would be perfect if the potential leads could be 

 made coincident with R. In reality, in the shunts described 

 below, about 90 per cent, of the self-induction effect is eliminated. 

 In carrying out this scheme of construction at the National 



FIG. 65. Non-inductive shunt. Designed at National Physical Laboratory. 



Physical Laboratory, the strip has been replaced by a manganin 

 tube enamelled on the inside. 



The tube is hard-soldered to hollow copper terminals and these 

 in turn are soft-soldered to the current leads, which are carried 

 back parallel to the tube, to about the middle of its length, where 



