62 Mr Palmer, Some Methods of Increasing 



If the potential is very nearly great enough to produce dis- 

 charge, then a spark taken from the positive terminal will start 

 the discharge, and under these conditions a spark could at times 

 be forced across a gap of 7 cms., but such an occurrence was 

 exceedingly rare, it being very difficult to take a spark with the 

 finger at just the right moment. 



I was more successful with a shorter spark-gap. The terminals 

 were pulled apart a distance of 15 cms. so that no discharge took 

 place when the machine was slowly driven by hand. When the 

 jars were charged the machine was stopped and the knobs brought 

 quickly together so that the distance between them was about 

 4 cms., then on taking a spark from the positive side a bright 

 discharge generally took place between the terminals. The 

 distance between the knobs could be more or less than this, 

 depending upon the time elapsing between charging and dis- 

 charging; if this was too small an interval, the spark passed as 

 the knobs were brought together, if too great, no effect was 

 produced on touching the positive side, but after one or two trials 

 the discharge was easily obtained. 



(3) By means of another Wimshurst Machine placed near. 



Hertz (Wied. Ann. XXXL, p. 983) has shown that discharge 

 between two terminals takes place at a lower potential when 

 the gap is simultaneously illuminated with ultra-violet light, and 

 it at first seemed that the effects of Expts. 1 and 2 might be 

 caused by the ultra-violet light produced by the small spark 

 taken from the negative terminal with the finger. The following 

 experiment shows that such is not the case. A large amount of 

 ultra-violet light was made by discharging a Ruhmkorff coil 

 between zinc terminals, and this light was focussed on various 

 parts of the spark-gap of the Wimshurst machine ; at the same 

 time Expts. 1 and 2 were repeated and no alteration in the 

 effects already obtained could be noticed — a small spark taken 

 from the negative terminal forcing discharge as before. 



Since there was a great deal more ultra-violet light produced 

 by the discharge between the zinc terminals than that between 

 the finger and the negative pole of the Wimshurst, it seems that 

 this form of radiation is not the cause of the increase in spark- 

 length. 



An interesting example of the effect produced by ultra-violet 

 light is the synchronising in the discharge of two Wimshursts. 

 Two machines were placed with terminals facing each other 

 about 40 cms. apart, and driven simultaneously ; the speeds were 

 then altered so that the intervals of time between successive 

 sparks were about the same for both ; it was then an easy matter 

 to make the two machines discharge at the same instant and 

 to continue doing this for a number of sparks. 



