Electrical Discharges in Air. 287 



difference of potential is necessary to sustain the disruptive 

 discharge ; and this variable difference of potential makes 

 itself evident as an apparent change of resistance. The arc 

 shortens or lengthens 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, this disruptive spark is oscillatory, even in the case 

 where the voltaic arc is produced by a dynamo-machine. 

 When w r e extend our studies to the forms of electrical dis- 

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

 discharge, 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 ordinary forms of lightning-discharge. I have lately 

 employed in connexion 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 9 to 10 inches long can be very con- 

 veniently studied by means of this apparatus; for a close 

 estimate of the difference of potential is possible, and the 

 spark-terminals do not change their sign during the ex- 

 periments. 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 accom- 

 panying reproductions, which fail, however, to give the details 

 of the negative. Fig. 2 is a photograph of a spark taken with a 

 euryscope-lens, such as is commonly employed for landscape- 

 work; this does not show any detail. Figs. 3, 4, and 5 

 are photographs taken with a Dallmeyer portrait-lens without 

 a diaphragm, and show, on the negatives, what may be con- 

 sidered an aureole accompanying the spark its entire length ; 

 furthermore, the oscillatory nature of the sparks is shown by 

 forked discharges which diverge from the main path of the 

 spark, and which point in opposite directions on the same 

 spark. If a photograph of lightning could be obtained which 

 would show a similar phenomenon, there could be no doubt 

 of the oscillatory nature of ordinary lightning-discharges. 



Since one can with a large number of Plante cells in con- 

 nexion with a rheostatic machine control the sign of the 

 electric charges on the spark-terminals, I was interested to 

 test the question whether the eye can detect any direction in 

 electric sparks. One observer looking through an opening 



