45 



two 100 and 1,000 respectively, so that they are read off 

 like a gas meter, and thus a resistance from 1 ohm to 

 11,110 ohms can be read directly, and by mounting the 

 commutator and the permanent arms of the Wheatstone 

 bridge on one board, we get a very compact instrument, and 

 have all the handles within easy reach for rapid change. 

 About 7 centimeters will be found ample for the diameter of 

 the disk, and the whole apparatus may be mounted on a 

 board about 45 centim. long and 10 centim. wide. 



The arrangement of resistances is much the same as in 

 slide resistances, and the plan of arranging these in a circle 

 lias been used for medical purposes, but I am not aware of 

 the resistances themselves being made to revolve thoutrh I 

 have not had any opportunity of investigating all the plans 

 previousl}^ adopted. It seems to me, however, that in cases 

 where only amateur or imperfect workmanship is available, 

 that this will be found the simplest plan, and also, I think 

 that when compactness and rapidity of action are important 

 this form may often be found useful, and, therefore, describe 

 it although there is no new principle involved. 



One such disk may also be used when a galvanic current 

 is being applied for medical purposes, in which case the 

 current is made to first pass through a high resistance of 

 several reels, and then without contact being broken the 

 resistance is brought down to null. In such cases it may 

 be found advisable to make the first resistance much lower 

 than the last. 



