1898.] on photographic plates by electric discharges. 



59..n 



discharge from it and the letters by the minute discharges from 

 their edges. Under the parts which touch the film there is no 

 action, and under the level parts which did not touch there is 

 weaker action. 



We also get a very good impression of the coin if it is laid 

 on the film and covered with an ebonite plate and the discharge 

 allowed to pass to the ebonite ; with this arrangement we have 

 an induced discharge around the coin which gives the impression. 

 If the ebonite is between the coin and the film no effect is 

 produced. 



All these effects on photographic plates would seem to be due, 

 in great part at least, to the light of the discharge, although the 

 amount of light may be very small indeed. 



That the positive and negative figures differ so radically is 

 due to the different forms of discharge in air in the two cases ; 

 the positive is more in the form of a brush and the negative of a 

 glow. 



Why the nature of the discharge into air from a positive and 

 a negative point should be so different is a property as yet not 

 understood. 



It is interesting to compare these photographic figures with 

 the figures produced by sprinkling a mixture of sulphur and 

 minium on an ebonite plate on which a discharge has passed. 

 The negatively electrified sulphur adheres to the plate if the 

 discharge has been positive, the positively electrified minium if 

 the discharge has been negative. 



The positive and negative figures thus produced are very 

 similar to the photographic figures, only the latter show more 

 detail. Here again when the pressure of the air surrounding 

 the plate is reduced the effect is similar to that with the 

 photographic plate; the radiating lines in the positive become 

 fainter and broader as the pressure is diminished. This effect 

 of the pressure of the air has been observed by Joly (Proc. Roy. 

 Soc. Vol. 47) who concludes that the figures owe their form chiefly 

 to the manner in which the discharge spreads in the surrounding 

 air. 



In experiments with these dust figures one often gets a 

 secondary positive in the centre of the primary negative or vice 

 versa, but this is simply due to a secondary discharge back from 

 the ebonite to the point used to convey the discharge to the plate. 

 In fact if we touch with an earthed metal rod a plate of ebonite 

 which has been electrified by a discharge having passed to it, 

 we get a figure around the point touched opposite to the sign 

 of the charge on the plate. 



In conclusion I wish to thank Prof. Thomson for the assistance 

 afforded by his suggestions. 



