BABBITT. 



THE VON WALTENHOFEN PHENOMENON. 



261 



The Investigation of Toroid III. 



The amount of iron in toroids I and II and as a consequence 

 the time required for the building up of the current are, even for 

 low voltages, comparatively small. Toroid III, on the other hand, 

 supplies conditions more nearly comparable to the large, solid mag- 

 netic circuit used at first. With an applied voltage just sufficient to 

 produce a medium flux den- 

 sity in the iron, approxi- 

 mately two seconds would 

 elapse before the ampere- 

 meter in the primary circuit, 

 Figure 3, would come to rest. 

 To make sure that no small 

 effects were masked by the 

 size of I and II, III was 

 tested in the same maimer. 



The circuit. Figure 3, re- 

 mained unchanged in every 

 respect save one, the ballistic 

 galvanometer. The slow flux 

 change in III required the 

 use of an instrument having 

 a much longer period. Re- 

 course was had to one of the 

 types used by Professor 

 Peirce *• ; the new galvan- 

 ometer constructed differed 

 from the old in that the balls 

 supported by the projecting 

 rods were replaced by an 

 aluminium disk weighted at 

 the circumference by a brass ring, Figure 19 (Plate). The period of 

 this heavy system was approximately five minutes ; and the error intro- 

 duced when the flux change was continuous and uniform for 20 seconds, 

 0.7 of one per cent; that is, the observed throw would be 0.993 times 

 the true throw. 



The same care in manipulation that applied to the earlier instru- 

 ment was necessary here. In fact the task of bringing the suspended 



Figure 19. The value of B is 3000: 

 the X indicates 24-step reversals, the O 1- 

 step reversals. 



" These Proceedings, 44 (1909). 



