THE STREAMERS IN THE ELECTRIC SPARK. 



75 



that of the line joining the spark to the rotating mirror, and a series of photographs 

 was taken with ziuc and with magnesium terminals. If the streamers are charged 

 atoms in motion, the theoretical effect to be expected would be that those going up 

 should be deflected in one direction and those going down in the opposite one, thus 

 producing in the photograph a want of symmetry in the course of the streamers from 

 the two electrodes. Such a want of symmetry was indeed found, but the deflections 

 were in the opposite directions to those which would be produced by the action of a 

 magnetic field on negatively charged atoms. The sparks in figs. 3, 4 and 5 were taken 

 in a magnetic field, and they show, in addition to the oscillations already referred to, 

 the want of symmetry in the streamers produced by the field. A close inspection 

 shows also that it is due to the streamers coming off not from the ends, but from the 

 sides of the electrodes. Examination of all the photographs taken 

 under these conditions showed that the effect which really takes place, 

 although there are occasional irregularities, is this : Suppose the top 

 electrode A (in the accompanying diagram) is the cathode, and B the 

 anode, for the initial discharge which gives the air lines, and that the 

 magnetic field is downwards through the paper. The. line of the first 

 discharge lies straight across from the point of B to the point of A, and 

 a streamer appears at the point of A. Then, during the back discharge 

 when B is the cathode, the streamer starts from the left-hand side of B, 

 not its point, but the anode is the point of A. The anode can lie 

 distinguished in the photographs as faintly luminous. The next 

 streamer starts from the right-hand side of A, the corresponding anode 

 for the discharge being the point of B ; and these paths with occasional 

 irregularities are adhered to in the subsequent discharges. 



The effect may, I think, be explained without difficulty. In the interval between 

 the first and second discharges the column of ions in motion which marks the line 

 of the first discharge is moved by the magnetic field to one side of the direct line 

 joining the electrodes. In the next discharge the line of maximum electric force is 

 directed towards this ionised column, and the streamer starts from the side of the 

 electrode in the direction of the maximum electric force. This gives an apparent 

 deflection opposite to that which would be looked for as the effect of the magnetic 

 field on the streamer itself. It is very possible that the latter exists when the 

 streamers are once in motion, but I have not been able to find any decisive evidence 

 of it in the photographs ; the effect, if existent, is overlaid by the drawing out of the 

 streamers by the mirror. The current itself in the second discharge is, however, 

 certainly deflected by the field, the anode being clearly marked at the point of A ; 

 thus the ions which carry the current must be deflected by the field along the curved 

 path CD. By the time the next discharge occurs the column of ions has been carried 

 to the right-hand side of the electrodes, and we get the streamer starting from the 

 side of A and the current traversing the deflected path EF. 



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