406 RECENT PROGRESS IN PHYSICS. 



The quotient ^r- is therefore : 

 For feeble ignition — 



I 



This series therefore confirms the results we obtained from the ei' 

 periments with the platinum wire. 



With copper wire the following results were obtained : 



§ 58. Comparison of the laws of galvanic ignition with those of Lei 

 for the devel >pment of heat. — According to the laws of Lenz, the quai 

 tity of heat liberated in a metallic wire increases proportionally 1 

 the square of the force of the current, and to the resistance to coi 

 duction of the wire. But with equal length the resistance to condun 

 tion is inversely proportional to the square of the diameter. I 

 therefore — all the other conditions remaining unchanged — the fore 

 of current increases as the diameter of the wire, the quantity of lies 

 developed must remain the same. 



But if in a thicker wire just as much heat is evolved as in a thinm 

 one, we should certainly expect that the former would not attain tl: 

 same degree of ignition as the latter, because the thicker wire imparl 

 more heat to the surrounding air ; therefore, in order to obtain a 

 equal degree of ignition in a wire of w times the diameter, we shoul 

 have to employ a current more than n times stronger, while accordin 

 to the above experiments a current with n times increased force 

 suflicicnt. 



Let us more accurately determine this relation. According to tl 

 researches of Lenz, above discussed, the heat produced in metall 



'"■ Melted after a while. 



