470 



PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



insulation test made between them. This applies to each of the six coils forming the 

 current weigher, arrangements being made whereby the six No. 1 wires may be 

 connected together, the six No. 2 wires similarly grouped, and the insulation between 

 adjacent wires of the whole instrument tested simultaneously. Any leakage between 

 the two adjacent helices can thus be readily detected and localised and remedied. 



Each of the fixed cylinders carries four helices, two upper and two lower, and each 

 suspended cylinder two. There are therefore twelve helices in all, and these are connected 



Fig. 4. General view of instrument, showing outer coils lowered. 



in series in the normal use of the current weigher by means of small concentric cables 

 running to a plug board and commutators outside the balance case. Flexible con- 

 nections are used as leads and returns to and from the suspended coils. The 

 commutators enable the direction of the current in any coil to be changed at will. By 

 reversing the current in the coils on the fixed cylinders the forces between the fixed 

 and suspended coils are reversed, and the apparent change of weight thus produced 

 is a measure of the square of the current used. 



