Observations on the Spark Discharge. 



69 



opposite sign is located. Thus, near the centre of the positive 

 pattern the cloudy- edged negative pattern is found, and vice versa. 

 More than one internal figure may sometimes, but rarely, be observed. 

 Thus a third figure, corresponding to the external pattern, may 

 occur. "Whether these secondary figures are due to a certain 

 amount of oscillation in the current flowing through the circuit 

 or not is not well determined. An experiment in which both the 

 capacity and self induction of this circuit was increased by inserting 

 in it the secondary circuit of a second coil of nearly equal dimensions 

 afforded figures no way differing from those previously produced. It 

 is observable, too, that very large and very small coils give a similar 

 arrangement of the figures. There are other reasons for believing 

 that the major part of the current in a coil discharge is unidirectional, 

 but a small amount of return current might cause the central disturb- 

 ance on the plate. Again, the central figure may be due to a back 

 discharge from the electrified surface of the plate, as Professor 

 Fitzgerald suggests. 



If the inner coats of two Leyden jars be connected with the poles of 

 the coil and so arranged that they can spark to each other across a 

 gap of 4 or 5 cm., the outer coats being separately connected with the 

 wires touching the powdered surface of the plate, the phenomena of 

 discharge differ somewhat. This circuit will afford an oscillatory 

 discharge, and accordingly it is found that triple figures are most 

 frequently formed and that the secondary figure is more conspicuously 

 developed. When the poles on the plate are very near each other, so 

 that a very vigorous spark passes, the secondary figures are seen to 

 encroach on the primary, even branching through them and mixing 

 with them, so that it becomes difficult to distinguish the + from the 

 — pattern. With a wider sparking distance the secondary are indeed 

 contained within the primary, but closely border upon them. And as 

 the sparking distance widens, the secondary figures retreat inwards 

 towards the centres. Finally, when the distance is increased to such 

 an extent that no spark passes, the central figure becomes very incon- 

 spicuous ; the -f- becomes very much to resemble the straggling lines 

 of the Lichtenberg figure, and the — becomes an irregular cloud. In 

 these observations it is seen that where the conditions for a vigorous 

 oscillation of the current are favoured the multiple character of the 

 figures grows more marked. 



(3.) Various powders were tried in the production of these figures 

 — charcoal, French chalk, very fine emery, mixed sulphur and red 

 lead, the protoxide and peroxide of tin. The last two powders, the 

 oxides of tin, differed in some respects in their behaviour from the 

 others. They exhibited, indeed, faintly, the types of pattern observed 

 with the use of lycopodium, but took in addition a ring-like formation 

 round the poles, the rings being irregular and wavy and sometimes 



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