44 



the adjoining German silver projection. The disk is revolved 

 by means of a bone or ebonite handle b, and these bosses are 

 thus brought against a strong spring (sp) up which the 

 current is led. If the flange connected with the axle is 

 brought against this spring then there is practically no 

 resistance, but if any other Hange is against the spring then 

 the current must pass through one or more reels of resistance. 

 As figured it would go through two reels of 10 ohms each, 

 and if it went through all the reels we get a total of 100 ohms. 

 As arranged, one boss does not leave the spring until the 

 next is in contact. 



The complete instrument consists of four such disks 

 similarly mounted and put into connexion, and on the first 

 disk the reels are 1 ohm, on the second 10, and on the other 



