RECENT PROGRESS IN PHYSICS. 



401 



mon, that the rise of temperature is proportional to the square of the 

 ;pantity of electricity and to the resistance of the conductino; wire. 



§56. Ignition of metallic loires hy the galvanic current. — While the 

 phenomena of the ignition of metallic wires by means of the discharge 

 of the Leyden jar have been elucidated by the ingenious researches of 

 Riess, corresponding investigations are wanting in reference to the 

 galvanic current, though the latter might probably ofier less diffi- 

 culties than the former. 



In Casselmann's treatise (already mentioned) " On the galvanic 

 carbon- zinc battery, Marburg, 1844," the following remark occurs on 

 page 43 : 



"A platinum wire of considerable length used for closing the 

 circuit, does not become red hot, but when shortened to a certain 

 length it does. Lessening this, however, by shortening it more and 

 more it reaches finally a length at which it does not become red hot 

 any more, and from this it follows that the ignition of the closing 

 wire reaches a maximum only when its resistance to conduction bears 

 a certain proportion to the quantity of electricity forcing its way 

 through it." 



If the current of a battery makes a wire red hot by passing through 

 it, still the force of this current must increase by shortening the wire, 

 and it therefore appears not quite probable that the stronger current 

 should no longer heat the shorter piece of wire to redness. To throw- 

 some light on this point, I made a series of experiments myself, since, 

 as above remarked, no thorough investigations have been made on this 

 law of ignition in the galvanic current. 



My experiments were made in the following manner: In the circuit 

 of the battery S (fig. 48) there was inserted at H, a wire-holder, 

 which will next be described, and at i>, a tangent comnass. At Q 

 there was a little mercury cup, by means of which the circuit could 

 readily be opened and closed. 



The wire-holder is represented in fig. 49. Upon a board two brass 

 rods were fastened, on each of which were two screw clami>,s capable 

 of sliding up and down. 



Fiff. «. 



Fig. 48. 



26 s 



