﻿490 Mr. J. B. Henderson on the Effects of Magnetic 



distance-pieces, and to each ring one of the pole-pieces was 

 soldered, the inner surfaces of the rings being conical to 

 receive them (see fig. 4). The distance-pieces were not at 

 opposite ends of a diameter of the rings, but one was dis- 

 placed a few degrees round the circumference of the ring to 

 allow of the introduction of the spiral and the ballistic coil, 

 which turned about the same horizontal axis and in the same 

 vertical plane (see fig. 2). 



Fig. 2. 



The ballistic method was used to measure the field, the 

 galvanometer being of the form designed by du Bois and 

 Rubens'*, used with the four 20-ohm coils in parallel. It was 

 standardized by means of the induced current produced in a 

 fine coil placed at the centre of a long straight solenoid of 

 thick wire, when a current through the latter was made or 

 broken f. The constants of both coils being known and 

 the current in the solenoid measured, the induced current 



* Wied. Ann. xlviii. p. 234 (1893). 



t For particulars of apparatus see Lehmann, Wied. Ann. xlviii. (1893 ). 



