RECENT PROGRESS IN PHYSICS. 449 



resistance ; or, in other words, which should produce the same re- 

 tardation of the discharge as that caused by the constant part of the 

 conducting circuit. 



Since the quantity of heat set free in a piece of wire is proportional 

 to its retarding force, and since, moreover, the heating of a given 

 wire in any part of the circuit can he determined hy aid of the elec- 

 trical air thermometer, we can compute the quantity of heat set free 

 in the whole circuit, were it to consist of a single wire of the length 

 L -f- /I and of a given thickness. Vorsselmann de Heer assumes that 

 in the whole circuit a quantity of heat, exactly equal to that com- 

 puted, is actually set free, because the circuit has the same retarding 

 force as the computed length of wire, and the heat set free is propor- 

 tional to the retardation. 



Biess, however, protests against this conclusion, (Pog. Ann. XLVIII, 

 320,) and with justice replies that the greatest part of the retardation 

 in the conducting circuit is due not so much to the continuous metal- 

 lic parts themselves as to the places at which they are joined ; and 

 that experiment gives us information as to the relation between the 

 retarding force and development of heat for continuous wires only, 

 but not for discontinuous wires when joined together ; that as yet we 

 know nothing of the relation between the retarding force and heating 

 at the joints. 



§ 51. Ignition and fusion of metallic wires by electrical dis- 

 charges. — While feeble currents, discharged through thin wires, pro- 

 duce changes of temperature, the laws of which Biess has thoroughly 

 studied, and with which we have hitherto been engaged, more powerful 

 discharges bring the wires into a state of ignition and even of fusion. 



The question now is, whether these effects, namely, the ignition 

 and fusion of wires, can be explained by the increase of heat accord- 

 ing to the laws found for lower temperatures or not. 



Biess has accurately investigated the ignition and fusion of metallic 

 wires by electricity, (Pog. Ann. LXV, 481,) and shows that this is 

 not the case. 



When a thin platinum wire 15 lines long, together with a thicker 

 one in the air thermometer^ were introduced into the conducting cir- 

 cuit of a battery, observations with feeble discharges gave, accord- 

 ing to the above laws for units of charge, a rise of temperature in the 

 thin wire of 0°. 68. 



By discharging the quantity of electricity, 42 in 5 jars, the wire 

 was completely melted. Computing the rise of temperature in the 

 thin wire for this charge, according to the known laws, we get 



0.68421 = 245°. 



This temperature is not high enough for the ignition, far less for 

 the fusion of platinum ; hence, it is clear that the temperature of 245° 

 which was computed according to the laws obtained for weak charges, 

 is not that to which the platinum really reaches when melted by 

 electricity. 



From this it follows that a powerful charge acts in a different 

 manner upon the wire than a weak one ; and it also appears that a 

 powerful discharge produces mechanical effects in the wire, which are 

 not at all shown by weaker dischargePr 



29 s 



