THE STKEAMERS IN THE ELECTRIC SPARK. 81 



represented by the locus of the extremities will be different from that of the streamers 

 themselves. There is also another reason why the velocity of the vapour in the 

 inductionless spark should be less than that of the individual streamers. We have 

 already seen (p. 74) reason to believe that the metallic vapour in the streamer is 

 electrically charged, and consequently its motion is alternately accelerated and 

 retarded by the oscillating electric force of the spark. In the spark with sufficient 

 self-induction the streamer can travel as far as the centre of the spark gap 

 during the period the accelerating force only is in play ; in the inductionless spark 

 the retarding force will come into play when the streamer has proceeded only a 

 minute distance from the electrode, and its average velocity throughout its whole 

 course will be accordingly diminished. The measurements of the velocities are in 

 entire accordance with these considerations. They show that the velocities of the 

 vapour for the inductionless spark are consistently only about half as great as those of 

 the individual streamers of the same metal in the spark with self-induction. 



Messrs. SCHUSTER and HEMSALECH found as a result of their researches that, 

 among the metals they examined, those which possessed the lowest atomic weights 

 gave the highest velocities in the spark. My observations do not confirm this 

 conclusion for the more extensive list of elements which. I have examined. For 

 example, sodium, having the lowest atomic weight, shows the lowest velocity of all the 

 metals examined. 



I have examined all my photographs in order to find, if possible, evidence of any 

 differences in the velocities which correspond to the different lines of the same metal 

 in the spark, but I have not been able to find any in any case. There are occasional 

 apparent differences, but they can always be adequately accounted for as the result of 

 some portions of the streamers by reason of under-exposure failing to affect the 

 photographic plate. 



It is to this cause, I think, that the differences obtained by Messrs. SCHUSTEK and 

 HEMSALECH in the velocities of the different lines of bismuth and cadmium must be 

 ascribed. It must be remembered in this connection that the intensity of a streamer, 

 while it may be very great near the poles, falls off rapidly as the centre of the spark 

 gap is approached. Thus the photographed outline of even a single streamer cannot 

 accurately coincide with the actual outline of the luminous vapour, for if we go 

 sufficiently far out the light can not be enough to affect the plate. This effect may 

 indeed be seen in almost every photograph, although in spite of it it is usually 

 apparent that the velocities are the same. A more marked effect will, however, be 

 produced in the photographs of the spark which has no inductance in circuit with it 

 beyond that of the connecting wires. Here, through the minuteness of the periodic 

 time of the oscillations, we are dealing with a number of streamers which are very 

 nearly (although not exactly) superposed on each other. The resulting image on the 

 plate is due to the combined photographic effect of them all. Let us suppose that the 

 brightness of one of the single streamers falls off at such a rate that, at a certain 



VOL. CCIX. A M 



