442 PROCEEDINGS OP THE AMERICAN ACADEMY. 



The effects in air of a voltage of three milliou are extremely suggestive 

 of the great energy in ordinary flashes of lightning. This voltage will 

 produce sparks six to seven feet in length in ordinary air between small 

 spheres. These discharges closely resemble lightning discharges. They 

 can be obtained of nearly three feet in length from one terminal of the 

 machine, the other terminal being insulated, by bringing near this 

 terminal a conductor which is connected with the ground. It is evident 

 that the electrical circuit is completed through the air to the insulated 

 terminal. When the apparatus for discharging the Leyden jars or 

 Franklin plates was first set up, the coated surfaces of these plates were 

 not more than a foot from the floor. It was speedily discovered, when 

 the room was darkened, that there was a powerful brush discharge to the 

 floor. The entire apparatus was then lifted three feet. In this case 

 there was a gain in the length of discharge which could be obtained ; 

 for the air resistance was thus increased. The apparatus is now pecu- 

 liarly well situated, being three feet above the floor and at a distance 

 from the walls of the room. When, however, the discharge takes place, 

 one feels as if a window had been suddenly lifted, letting in a gust 

 of wind, one's coat lifts, and sparks can be drawn from the neighboring 

 walls. In order to get the full effect of such a voltage in producing dis- 

 charges, the apparatus should evidently be placed thirty or forty feet above 

 the earth, and should be remote from other masses. 



Photographs of these powerful discharges closely resemble those 

 taken of lightning. It is evident to my mind that many of the pecu- 

 liarities of lightning flashes are due to imperfection in lenses, or to a 

 want of focus. The so called ribbon discharge can be closely imitated 

 by putting the camera out of focus. When a portrait lens of large 

 aperture is employed, many details are obtained which are not shown 

 by the ordinary landscape lens. Thus Plate III., taken by a Dallmeyer 

 portrait lens, represents a single discharge from a sphere one foot in 

 diameter to a long linear conductor which formed the positive terminal. 

 The main discharge was between four and five feet long, and the air 

 between the terminals was filled with a powerful brush discharge, which 

 passed off at right angles to the surfaces. The portrait lens also shows 

 that a single discharge is really made up of to and fro oscillatory dis- 

 charges; for bifurcations are seen pointing in opposite directions on the 

 line of discharge. 



The effect of the high electromotive force which can produce such 

 powerful discharges in air is also of great interest when it is examined 

 in rarefied gases. The spectrum of hydrogen does not appear to be 



