Observations on the Spark Discharge. 



77 



a jar, are passed over emery dusted on glass. The phenomenon recalls 

 the shallow penetration of transient currents in conductors, and sug- 

 gests that possibly the inflow of energy from the surrounding medium 

 is not, in these cases of heavy sparks, concentrated to form linear dis- 

 ruption, but disruption on a cylindrical surface. 



(13.) To investigate in some measure the effects of the discharge in 

 the substance of the dielectric plate, a plate of ordinary glass was 

 employed, having its upper surface flooded with melted paraffin, kept 

 fluid by resting the glass on a warm sheet of metal. During discharge 

 a ridge or mound of the liquid paraffin was repelled into the region 

 between the poles extending nearly across the plate, and always nearer 

 to the negative pole. Around the poles the paraffin was repelled as if 

 by a strong wind, leaving a circle of glass clean and bare. The circle 

 was largest round the positive pole. Beyond this a radial puckering 

 of the surface of the fluid could be observed. By allowing the glass 

 to cool while continuing the action of the coil, these phenomena could 

 be examined in the solid paraffin. It was curious to observe that as 

 cooling progressed isolated heaps of paraffin gathered from the sur- 

 rounding fluid, principally near the negative pole, when cold standing 

 5 or 6 mm. above the general level — possibly a surface tension effect. 

 Short lengths of finely cut up copper wire scattered over the plate 

 while the paraffin was still fluid gathered towards the poles, and, 

 with much gyration and oscillation, set themselves radially to the 

 poles and along lines of flow between the poles. In many cases they 

 struggled into the vacant ring close to the pole, and moved about till 

 they gradually built up lines reaching from the edge of the paraffin 

 nearly to the pole. In some places they set themselves into a kind of 

 network, attachiug themselves together, end to end, as if magnetised. 

 These appearances were observed on cooling. Very remarkable are 

 innumerable whirls and eddies to be found on the frozen surface, in 

 some cases so minute as only to be defined with a strong lens. 



Similar phenomena were observed using fluid paraffin oil. They 

 reveal the presence of stresses in the dielectric accompanying dis- 

 charges, which the rigid nature of the glass refuses to show, and 

 suggest the degree to which discharge takes place in the plate. 



Some Phenomena Occurring in the Path of Discharge. 



The degradation of energy taking place in an electric discharge 

 through a gaseous medium is complex in character. There is 

 development of heat, probably according to the laws of Ohm and 

 Joule ; kinetic energ} T imparted to the molecules by electrostatic re- 

 pulsion and attraction, and chemical potential energy in the liberated 

 ions. The liberation of ions cannot be supposed to occur in the same 

 manner as in liquid electrolytes. In the latter case the actual expendi- 



