Electrical Discharges in Air. 285 



The accompanying sketch shows the disposition of some of 

 the parts. It represents a section by a horizontal plane. 

 A is the condensing-lens, B the wedge, C the negative tem- 

 porarily cemented to B by fluid turpentine, D the speculum. 



XXXVII. Electrical Discharges in Air. 

 By John Trowbridge *. 



THE flaming discharge from a large accumulator with its 

 nucleus consisting of a dazzling white spark is evidently 

 a form of voltaic arc ; and I was interested to discover, if 

 possible, the mechanism, so to speak, of the voltaic arc. Does 

 it follow Ohm's law in respect to resistance, and is there an 

 oscillatory phenomenon ? It is well known that electric 

 sparks can be greatly increased in length by interposing a 

 gas-flame between the terminals of a Ruhmkorf coil or by 

 moderately rarefying the air between such terminals. The 

 conditions in the voltaic arc favour a greatly increased length 

 of a disruptive spark between the positive and negative 

 carbons. This can be seen in the photograph of the arc pro- 

 duced by a high-tension accumulator: and doubtless the 



* Communicated bv the Author. 



