Machine for producing High Tension Electricity. 181 



with comparative feebleness to cross over, and only reached the 

 positive wire in scanty fragmentary portions. These appearances 

 were strongly suggestive of a dominating force emanating from the 

 positive side and opposed by a weaker force of the same nature in the 

 opposite direction. The oscillations which are known to attend every 

 disruptive discharge necessarily involve passages from the negative 

 as well as the positive side. In fact the spark must be regarded as 

 consisting of a dying out alternating current of prodigious frequency 

 in which the positive alternations have f the ascendency over the so- 

 called negative ones ; the excess constituting the available current. 



Although the arc flame presented no appearance of flowing motion, 

 I thought it necessary to test the question of any movement in the 

 longitudinal direction by bringing the arc in contact with light 

 powdered substances. For this purpose I placed a small heap of 

 chalk dust upon a plate of mica, and caused the arc flame to pass 

 through it, but none of the dust was moved lengthways. The dis- 

 placement was entirely lateral, the material being neatly hea/ped up 

 on each side in a ridge, which curved inwards towards the points of 

 the electrodes, beyond which no displacement was visible. The action 

 appeared to be that of a gentle push, rather than that of a sudden 

 impulse ; but when sparks passed instead of flame, a scattering force 

 was exerted, which operated sideways, and not in the track of the 

 spark. In this case I observed that the scattered dust showed a 

 tendency to settle upon the plate in curved lines and symmetrical 

 figures. I followed up this hint by sifting fine black dust upon 

 white cardboard and passing sparks over the surface, and I thus 

 obtained most unmistakable proof of symmetrical arrangement. 

 This, however, was only a crude method of procedure, and it required 

 a great deal of perseverance and innumerable trials before I suc- 

 ceeded in producing the effect in a satisfactory manner. At length, 

 however, I was enabled to produce perfect dust figures which pre- 

 sented pictures of the disruptive discharge revealing the existence of 

 forces of which the eye could otherwise take no cognizance. Many of 

 these figures I got photographed on the spot, and a selection of them 

 accompanies this paper. They represent the effect not of a single 

 discharge but of a succession of sparks, generally beginning with a 

 light one and gradually increasing the power as the dispersion pro- 

 gressed. To put all the power into one discharge had too much 

 dispersive effect and produced great irregularity in the figure. A 

 perfectly even sifting of the dust was also essential to a good result, 

 and this could only be effected by the entire exclusion of air draughts 

 and a regulated action of the sieve. The best kind of dust to be 

 used and its proper degree of fineness were also matters requiring 

 many experiments to determine. The dust which I ultimately used 

 consisted of calcined magnesia worked up in a mortar with a sufii- 



