J. Trowbridge — Electrical Discharges in Air. 191 



number of oscillations in the circuit containing the arc and the 

 condenser. A curve was then plotted with the number of oscilla- 

 tions as ordinates and the ohmic resistance of the circuit as 

 abscissas. It was thus found that the 

 apparent resistance of the voltaic arc was 

 equivalent, in the case I considered, to a 

 resistance of eight-tenths of an ohm (*8 

 ohm). It was found, moreover, that an 

 arc one-quarter of an inch long did not 

 present more resistance than one, one-half 

 an inch long. The apparent resistance, 

 therefore, of the voltaic arc does not fol- 

 low Ohm's law. I am led to believe that 

 the mechanism, so to speak, of the voltaic 

 arc is as follows : A disruptive discharge 

 accompanies a flaming discharge, and 

 serves as a species of pilot spark. A 

 variable difference of potential is neces- 

 sary to sustain the disruptive discharge ; 

 and this variable difference of potential 

 makes itself evident as an apparent change 

 of resistance ; the arc shortens or length- 

 ens in obedience to the mechanism of the 

 lamp which is employed. 



A family resemblance may be said to exist between all forms 

 of electrical discharges in air. Thus in the voltaic arc we 

 have a disruptive discharge combined with a flaming discharge. 

 In general the disruptive spark is oscillatory even in the case 

 where the voltaic arc is produced by a dynamo machine. 

 When we extend our studies to the forms of electrical dis- 

 charges which are free to a great extent from the flaming dis- 

 charge, such as the disruptive sparks from electrical machines, 

 Tesla and Thomson transformers and the Plante rheostatic 

 machine, we are struck by their close resemblance to the ordi- 

 nary forms of lightning discharge. I have lately employed, in 

 connection with five thousand Plante cells, a Plante machine 

 with thirty condenser plates made of glass, one-sixteenth of an 

 inch in thickness, with a coated surface of 15x18 inches. 

 Sparks nine to ten inches long can be very conveniently 

 studied by means of this apparatus, for a close estimate of the 

 difference of potential is possible and the exciting apparatus 

 does not change its sign during the experiments. To the eye 

 each spark seems to be surrounded by a bright radiance or 

 aureole of which it appears to be the nucleus. In order to 

 ascertain whether this radiance was an actual phenomenon. I 

 employed a portrait lens of large aperture, and some of the 

 results are exhibited in the accompanying reproductions, which 

 fail, however, to give the details of the negatives. Fig. 2 is a 



