of the Electric Spark. 287 



the centre of the spark-gap. Dark spaces at the electrodes 

 separate the oscillations from each other and serve to 

 measure the period o£ the circuit. At the electrode whose 

 potential is, for the time being, positive the luminosity of 

 the streamers is of longer duration than at the negative 

 electrode, and hence the streamers are not so distinct ; the 

 difference is not so marked as in the spark with small self- 

 inductance. 



When the self-inductance of the spark is increased, it is 

 seen that during a single oscillation several metallic streamers 

 start out from the electrodes ; they are more distinct with 

 magnesium than with lead, bismuth, mercury, or calcium. 

 A typical example is shown in fig. 4, PI. IV. The streamers 

 are too numerous and too close together to be due to 

 simple harmonic overtones of the fundamental period of the 

 circuit. 



Velocity of the Metallic I r apour. 



In the spark with small self-inductance it was seen in the 

 earlier paper that three or four oscillations have passed before 

 the metallic vapour has reached the centre of the spark-gap, 

 and that the velocity of the vapour produced at the com- 

 mencement of the spark was indicated by the slope of the 

 first edge of an envelope formed by the streams of the first 

 few oscillations. When the period has become great enough 

 for the metallic vapour first produced to reach the middle 

 of the spark-gap during the first balf-oscillation, the velocity 

 of this vapour is given by the slope of the first edge of the 

 streamer which starts from the electrodes at the commencement 

 of the spark. In the following table are given the velocities 

 of the metallic vapour produced at the commencement of the 

 spark for the different values of capacity and self-inductance. 

 The spark-length was constant, viz. 8 mm. 



A reduced temperature would explain the smaller velocities 

 obtained with larger self-inductance. It is possible, however, 

 that the amount of metallic vapour produced initially is 

 smaller than in the case of small inductance, and the 

 assumption that the velocity reaches its limiting value, the 

 velocity of sound, may therefore be no longer even approxi- 

 mately true. It has also been shown experimentally that a 

 smaller quantity of metallic vapour reduces the velocity *. 



It has been noticed in the case of the spark between 



* Royds, loc. cit. m 



