chap, xin.] RHEOSTAT OF WHEATSTONE. 



135 





68. Bheostat of Wheatstone. 



rheocord and compensator, which will permit of a 



perfectly definite amount of resistance being readily 



interposed in the way of a current without altering any 



of the wires. Such an instrument is the rheostat, 



invented by Wheat- _ ( 



stone. As shown in 



Fig. 68, it consists of 



two cylinders, one, CB, 



made of brass, and there- 



fore a good conductor, 



the other, AD, of wood 



(an insulator) with a 

 spiral groove cut in it. 

 On CB is wound a fine brass wire about 40 yards long, 

 though the instrument can be made of any size and 

 the wire of any length. The wire is partly wound 

 also on the wood, so that each turn, lying in its groove, 

 is insulated from its neighbour. The end of the wire 

 on the wood cylinder is connected with a binding 

 screw, and the metallic cylinder is also in connection 

 with a binding screw. Now let the -f- wire from a 

 battery be led to the screw of the metallic cylinder; it 

 and the wire coiled on it form a thick conductor, and 

 offer no resistance of any consequence. The pole 

 being connected with the wire wound on the wooden 

 cylinder, the current must pass from the metal 

 cylinder and traverse each turn of the wire wound on 

 the wood before it can pass off' at the binding screw. In 

 traversing this fine wire it meets with considerable 

 resistance, and the greater number of turns of wire 

 that lie in the spiral of AD, the greater is the 

 resistance. By turning the handle m, the wire may 

 be wound on to the cylinder AD and the resistance in 

 the circuit increased, or wound off on to CB and the 

 resistance diminished. By the dial on c the length of 

 the wire in feet and inches on AD can at once be 

 counted, and the resistance estimated. 



