634 Prof. A. Battelli and Mr. L. Magri on 



a spiral for high-frequency currents are wanting, we had to 

 apply to the experimental method,, which indeed led to 

 satisfactory results. 



(d) Resistance of Spark. 



37. Our experiments gave us also the means of deter- 

 mining the resistance of the spark, which nowadays forms 

 one of the principal problems connected with the electrical 

 discharge. Though it has been the object of some deter- 

 minations, it can not be said to be known with certainty, 

 and more especially it is not known how it depends on the 

 frequency of the oscillations. 



Biernacki* tested the resistance of the spark of a con- 

 denser by comparing it with that of an electrolyte inserted 

 between the balls of a resonator identical with the oscillator 

 itself, but this investigation is relative only to the sparks of 

 Hertz's oscillators, viz., to oscillatory discharges of high 

 frequency, where small quantities of electricity only are in 

 question. 



Kaufmann f, on the other hand, found that the quantity of 

 heat evolved in the spark by the discharge of a battery 

 of Leyden-jars is connected with the resistance W of the 

 remaining part of the circuit by the formula 



a B 



q= + w 



A and B being constants. Thus the resistance of the spark 

 is not comparable with a metallic resistance. 



Moreover, he found that the heat evolved in the spark 

 increases with the potential-difference more rapidly than the 

 total energy of the discharge ; that is to say, he found 



approximately ~ d = const., instead of — a = const. This latter 



result is not in accord with those derived from our ex- 

 periments. 



Besides, he found the quantity of heat evolved in the 

 spark to be larger (about double) between two zinc electrodes 

 than between two brass electrodes. We had for cadmium 

 and platino-iridium electrodes about the same values of Q 

 (see Table III.). 



More recent and accurate measurements are those of 

 Cardani J, who of course determined the apparent resistance 

 of the spark by deriving it from the total heat evolved less 



* Journ. de Phys. [3] iv. p. 474 (1895). 

 t Wi^d. Ann. lx. p. 653 (]897). 

 % N. Cim. [4] xi. p. 113 (1900). 



