200 Prof. J. C. PoggendorfF on the Induction Apparatus 



of the above assertion, the more so as former observations on the 

 sparks of frictional electricity have led either to no results at all, 

 or only to uncertain ones. 



With the induction sparks obtained without trou.ble from a 

 continually interrupted current, such a proof is without the least 

 difficulty. It is even furnished in a very perceptible manner by 

 a good mercurial thermometer. With this instrument of course 

 no measurements can be made, although comparative results can 

 be obtained by securing a continuous development of sparks, by 

 means of as uniform a motion and condition of the hammer as 

 can be possibly attained, as well as by paying attention to other 

 precautions which will be easily recognized as necessaiy. 



The following are some of the experimental results which were 

 obtained : — 



1. When a delicate thermometer is placed in or close to the 

 stream of sparks of the induction apparatus, a considerable rise 

 immediately takes place, contrasting greatly with the small de- 

 velopment of heat in the wire itself. This is the more remark- 

 able, because, according to Joule's law, the heat generated in a 

 given time throughout the whole circuit of the induction current 

 is smaller when the circuit is broken by air than when it is con- 

 tinuous. ■■■ 



2. All other things being equal, this elevation of temperature 

 is different according to the nature of the metals or bodies from 

 which the sparks issue. 



In this respect, however, platinum, copper, iron, and silver 

 differ little from one another, although amongst them sdver ap- 

 peared to act strongest, and platinum weakest*. But a great 

 difference exists between these metals and those which are but 

 little coherent, and are easily fused or vaporized. On an average, 

 their action is double that of the former. 



The following are the elevations of temperature observed during 

 the space of one minute in a series of experiments with different 

 metallic points. The cylindrical reservoir of the thermometer 

 was 11 millims. long and 3'5 millims. in diameter; its axis was 

 placed between the points from which sparks issued, and at a 

 distance of 2^5: millims. from each. 



Platinum .... 18| C. 



Lead 30| ... 



Tin 33 ... 



Antimony .... 34i ... 



Zinc 35 ... 



Bismuth . . . . 37 ... 

 The difference was still greater when the distance between the 

 * Graphite alone produced a still smaller degree of heat. 



