372 Scientific Proceedings, Royal Dublin Society. 



sufficiently thick to become a less imperfect conductor than the 

 film of mica. Successive sparks traversing this path deposit more 

 and more metal, until the conducting surface is sufficiently good 

 for the current to traverse it continuously, then at some point 

 where an inequality in the surface, or an imperfection in the 

 material of the non-conductor, admits of a perforation, and the 

 spark passes once and continues to pass subsequently. 



Effects caused by the Oscillation of the Electric Spark. 1 — 

 According to Feddersen, in the Paper already quoted in the 

 Introduction, the number of oscillations is estimated to be over 

 100,000, and often more than a million per second. The inertia 

 of the discharge causes it to overshoot the mark first in one 

 direction and then in another. This is quite in accordance with 

 certain facts which I have long since observed ; and recent ex- 

 periments of mine have shown that another proof of these oscil- 

 lations is easily obtainable. 



When photographs of the spectrum of the sparks from a good 

 conducting and easily-volatile metal, like zinc or cadmium, are 

 carefully examined, the negative electrode exhibits a series of long 

 lines, with a nimbus and an extension below the point of metal. 

 The positive electrode which is uppermost shows the same exten- 

 sion and nimbus, nothwithstanding that the spark is supposed to 

 pass from the negative to the positive only. If the positive elec- 

 trode is of some other metal the negative does not yield lines 

 which stretch quite across from point to point, or if so, the lines 

 have only one nimbus around the negative point. This nimbus 

 has always been attributed to the silent discharge which takes 

 place at every little roughness and irregularity on the electrode. 

 If, however, there is a nimbus at the upper or positive electrode, 

 this must be due also to a silent discharge of an opposite character. 



1 Professor Oliver Lodge, in a lecture delivered at the Eoyal Institution, has quite 

 recently (March 9th) demonstrated the character of the Leyden jar discharge. In 

 Nature (vol. xxxix. p. 471) he attributes the discovery to Joseph Henry, of Washing- 

 ton, and quotes from his writings, published as early as 1S42, from which the 

 following extract is taken: — "The discharge, whatever may be its nature, is not 

 correctly represented (employing for simplicity the theory of Franklin) by the single 

 transfer of an imponderable fluid from one side of a jar to the other ; the phenomenon 

 requires us to admit the existence of a principal discharge in one direction, and then 

 several reflex actions backward and forward, each more feeble than the preceding , until 

 the equilibrium is obtained.^ 



