408 KECENT PKOGRESS IN PHYSICS. 



kept unchanged. This difference already shows that the galvanic 

 ignition is essentially of another nature from that produced by the 

 discharge of the jar. 



§ 59. Determination of the voltaic combination required to produce 

 ignition in given metallic tvires. — The mean values above obtained for 



the quotient indicate the force of current necessary to bring a wire 



D 

 of 1 millimetre in diameter into the corresponding degree of ignition. 

 Therefore, for a platinum wire 1 millimetre in diameter, to make it 

 feebly red the force of current required is 165 ; to make it red hot the 

 force of current required is 172 ; to make it nearly white hot the force 

 of current required is 220. 



For an iron wire 1 millimetre in diameter to make it feebly red the 

 necessary force of current is 121 ; to make it red hot the necessary force 

 of current is 135. To make a copper wire 1 millimetre in diameter red 

 hot a force of current of 433 is required ; for silver this value is 432. 



I consider these numerical values only as first approximations. 



Denoting by s the force oi current which is required to bring a wire 

 1 millimetre in diameter to a certain state of ignition, th en s.c? indicates 

 the force required to produce an equal amount of heat in a wire of the 

 same metal whose diameter is d. 



If once we know the force of current a required to produce a 

 certain degree of ignition in a piece of wire of given diameter, and also 

 the resistance to conduction r, which this wire in connexion with the 

 other part of the closing circuit offers, then it is easily computed 

 what com- bination of voltaic elements, of a known nature, has to be 

 employed for the purpose. 



Let e denote the electro motive force, iv the specific resistance of one 

 of the cups employed. These have to be so combined that they form 

 a battery of n elements, each consisting of m cups placed together. 

 Now, the values of n and m are to be determined. 



The cups must be so combined that the resistance of the battery is 

 equal to that of the closing wire ; the total resistance, therefore, must 

 be equal to 2r. We have, therefore, 



ne 



1 2ra 

 and n =. • 



But the specific resistance of our battery is 



n 



■ w ■=. r. 

 m 



Therefore, m = lo ; 



and the value for n being substituted, 



™ 2ii;a 

 m = 



