the Spark-Length of the Wimshurst Machine. 



61 



poles or passed to the nearest portion of the framework of the 

 machine. 



When driven with uniform speed there was a limit to the 

 potential difference which could be obtained between the terminals 

 of the Wimshurst. Before the finger was presented to the negative 

 terminal the machine was run in each case until the jars were 

 charged to this maximum potential, and if this was not obtained 

 then the small spark taken by the finger did not produce a spark 

 discharge but only a brush discharge. 



It does not appear to make any difference how the spark 

 is taken, the finger, an uninsulated ball or knob of a small Leyden- 

 jar held in the hand, all giving similar results. I was unable to 

 produce any increase, however, by fastening a wire W (Fig. 1) with 

 ball F attached to the outer coating of the jar and taking the 

 spark with F from B. 







o o 



Q o- 



w 



Fig. 1. 



The conditions necessary for success are that the jars must 

 be on the machine, and their outer coatings in good metallic 

 connection. 



The effect can be produced with a single jar, large or small, 

 by standing it on paraffin wax or any other insulator and 

 connecting a coating to each terminal. 



(2) By taking a Spark from the Positive Side. 



In endeavouring to measure the increase noted above it 

 seemed that the effect produced with a gap of about 5 cms. or 

 less was the same whether the spark was taken from the positive 

 or negative side. This suggested the idea that the increment 

 could be produced by the former as well as by the latter, and 

 therefore I tried to obtain the effect on the Wimshurst. For 

 a long time I was unsuccessful with a spark-gap of 10 cms. or 

 more ; there seemed to be little effect by placing the finger near 

 the positive pole and taking a spark, and it was impossible to 

 produce a great increase in length like that obtained by the 

 method of Expt. 1. 



