Sparking Voltages in small gaps. 



465 



sparking values found (except Vi) will vary with changes in 

 the initial ionization of the gap. It is necessary now to 

 test the matter by -experiment. Two obvious checks present 

 themselves. In the first place, if the argument is true, the 

 sparking voltage for a given gap should vary with the 

 primary current at break in a manner shown at figure 2 



Fig. 2. 





Current in Primary 



which is obtained from figure 1. In the second place the 

 instant of occurrence of the spark should advance as i is 

 increased, and the amount of advance should progressively 

 diminish as i increases. This latter is easily tested. & Starting 

 with i as a minimum and focussing the cross wires of a tele- 

 scope on the image of the spark in a rapidly rotating mirror, 

 itwas_ found that the image shifted relatively to the cross- 

 wires in the manner anticipated. Regarding the variation 

 of Y with i 0i this was tested by an experiment on a small 

 gap formed between polished brass balls of *5 inch diameter, 

 both electrodes being insulated from earth. First the static 

 sparking voltages for different gap settings were found. 

 The results are shown by figure 3. Then using impulsive 

 voltages given by a 10 inch induction-coil, the sparking 

 voltage was found with the least current in the primary that 



