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XXXI. On the Existence of an Undxdatory Movement accom- 

 panying the Electric Spark. By Ernest H. Cook, 

 D.Sc. (Lond.), F.I.C., Merchant Venturers' School, 

 Bristol*. 



[Plate HI.] 



1. TF a powdered substance be scattered over a smooth 



J- plate, and the plate held near a pair of terminals 

 between which an electric spark is passing, the powder will 

 arrange itself in a more or less regular series of concentric 

 circles. The common centre of these circles (when the spark 

 is small) appears to be a point directly under that halfway 

 between the terminals. The general appearance presented by 

 these curves will be apparent by a glance at the figures in the 

 Plate which accompanies this paper. These are approximately 

 of the same size as the original. It will be noticed that the 

 figures roughly divide themselves into two classes — (1) those 

 with a clear space in the middle, and (2) those with a circular 

 space, within which the powder is apparently unaffected. 



The first class are obtained by holding the plate near to 

 the terminals, the distance varying with the energy of the 

 spark. The cleared space is irregularly elliptical, with its 

 major axis at right angles to the direction of the spark, 

 and the area of the space increases as the intensity of the 

 spark increases. The second class are formed when the 

 distance between the spark and plate is increased until the 

 elliptical space ceases to make its appearance. 



2. Mode of Obtaining the Curves. — The most convenient 

 arrangement and the one which gives the best results is as 

 follows : — An induction-coil has each of its terminals con- 

 nected with a Leyden jar, and with one of the handles of a 

 universal discharger. The plate with the powder scattered 

 over it from a sieve is held under the points at a distance 

 which can be varied at pleasure The distance between the 

 terminals, and therefore the length of the spark, can also be 

 easily adjusted. On connecting the coil to the battery a 

 succession of sparks of condensed electricity is obtained, and 

 the powders will be seen to arrange themselves in the manner 

 described. The first spark causes a distinct effect, but a 

 continuance is necessary to bring out the regularity of the 

 curves. 



Should it be required to fix the powders in the positions 



* Communicated by the Physical Society : read June 23, 1888. 

 The paper ^vas illustrated by a series of photographs: from two of 

 these the figures in the Plate have been engraved. 



U2 



