RHEOSTATE. 323 



in 



The rapidity of heating is then independent of the diameter, and 

 at 



the relative increase of resistance for unit time is 







dr _ dO _E 2 jx^ 

 ~dt~ CL ~di~~]~ 7^' 



For a copper wire we have ^ = 8*85, c= 0*095, 7 = 0*840, and 

 a = 0*0039; if> further, E = # volts = n io 8 , (7=1615, and J = 4.2. io 7 , 

 we have 



dd n 2 



This is the number, in centigrade degrees, by which, apart from 

 losses, the temperature of a wire would be raised in each second for 

 a fall of potential of n volts per centimetre. 



For argentan we have a! = a x o*ii and <r'= 13 a-, the value of y 

 being virtually the same ; it follows therefore that, other things being 

 equal, the heating would be 120 times less for argentan than for 

 copper. 



These effects are diminished by allowing the current to pass for as 

 short a time as possible through the coils. 



930, RHEOSTATE. Before boxes of coils were introduced, a 

 greater or less length of wire was brought into the circuit in order 

 to vary the resistance. Pouillet* used a platinum wire 132 metres 

 in length stretched backwards and forwards on a board. Wheatstone 

 constructed a more convenient form of the apparatus, and called it 

 the rheostate. 



Wheatstone's rheostate consists of two parallel 'cylinders of the 

 same diameter turning in the same direction, and with the same 

 velocity ; one is of brass with a smooth surface, the other of glass 

 or of hard wood with a continuous helicoidal grove. A wire of 

 argentan winds itself on either of the cylinders according to the 

 direction of the winding. All that portion of the wire which is in 

 the grooves of the wood is insulated and acts as resistance; that 

 which is on the brass forms part of it, and is as it were suppressed. 



This ingenious arrangement has many practical inconveniences. 

 The wire being drawn first in one direction, and then in another, is 

 modified or even loses its shape; its resistance can no longer be 



* POUILLET. Comptes rendus, Vol. iv., p. 785. 1837. 



t WHEATSTONE. Bakerian Lecture for 1843. Phil. Trans., -Vol. v., 133, 

 p. 303. Scientific Papers, p. 105. 



Y 2 



