THE STREAMERS IN THE ELECTRIC SPARK. 73 



camera while keeping control of the shutter, it was possible to expose for the few 

 seconds required to just catch one or two good images on the plate and yet avoid the 

 confusion of many overlapping ones. 



The appearance of the drawn-out spark obtained with a small inductance 

 (O'OOOl henry) in circuit is shown in the photograph, fig. 1, Plate 2. We see the 

 initial air discharge extending straight across from pole to pole, and succeeding it the 

 streamers, starting from the poles and crossing each other in blurred masses in the 

 centre of the spark gap. The streamer corresponding to each oscillation is observed 

 to be much more prominent at one pole than at the other ; this fact was noted both 

 by FEDDERSEN and by SCHUSTER and HEMSALECH, but they were unable to determine 

 from their photographs whether it was the positive or the negative pole from which 

 the main streamer started. SCHENCK,* who subsequently investigated the spark by 

 means of a rotating mirror, observed that the streamers came from the cathode in each 

 case. HEMSALECH,! in a later research on the spark in which he separated the, 

 oscillations by blowing a current of air across them, found that it was from the positive 

 pole that the streamers emanated. In each of my experiments the direction of the 

 first discharge of the spark was determined by noting the direction of the discharge 

 produced by the induction coil through an X-ray tube ; the nature of the poles in the 

 subsequent oscillations could then be easily ascertained by counting from the first 

 discharge. In all the photographs, which comprise different metals, inductances, and 

 capacities, and also different lines of the same element, I find, in agreement with 

 SCHENCK, that in every case it is the cathode from which the stronger streamer 

 emanates. There is, however, in all cases a tendency to a discharge from the anode. 

 In the softer metals, such as magnesium and lead, this becomes very pronounced and 

 nearly as strong as the cathode discharge. (See fig. 2, Plate 2, a spark between 

 magnesium terminals.) With regard to HEMSALECH'S result it is to be observed that 

 he worked with much more inductance in the circuit than either SCHENCK or myself, 

 and it is possible that his different result is diie to the conditions in his experiments 

 being more analogous to those obtaining in the arc than in the ordinary spark. 



An interesting point was noticed in several of the photographs which throws a clear 

 light on the nature of the streamers. This is illustrated in figs. 3,4, and 5 (Plate 2). 

 These photographs show oscillations of some of the streamers which represent backward 

 and forward motions of the vapour forming them which are exactly synchronous with 

 the oscillations of the spark itself.j 



* ' Astrophysical Journal,' vol. 14, p. 116 (1901). 



t 'Comptes Rendus,' vol. 142, p. 1511 (1906). 



| The sparks reproduced in figs. 3, 4, and 5 were taken in a magnetic field as described on p. 75. 

 The magnetic field, while having nothing to do with the production of these oscillations, generally makes 

 them more prominent, as the irregular course of the streamers produced by the field, which is referred 

 to on p. 75, prevents them overlapping in the centre of the spark gap. It thus happened that these 

 negatives showed the oscillations best, but the backward motion of the vapour is shown by many of my 

 negatives of sparks without a field. 



VOL. CCIX. A, L 



