the Spark-Length of the Wimshurst Machine. 65 



The following were the results obtained : 



(a) With an indiarubber ball as the positive terminal so that 

 a positive brush was formed, and with a spark-gap of 6 cms., the 

 slightest push of the glass rod produced a brush and very often 

 a spark discharge ; with one of 8 cms. a big brush discharge was 

 caused quite across the gap ; at 9 cms. and upwards the brush 

 divided into two parts, the positive being very large and extending 

 three-quarters of the way across. The spark obtained with the 

 shorter distance was very interesting, it would often pass from 

 the brass cup round to the point on the rubber and then straight 

 across the space between the terminals. 



(b) With the rubber ball as negative terminal and with a 

 spark-gap of 4 cms., a slight push to the glass rod produced a 

 brush and very often a spark discharge ; at 6 cms. a small nega- 

 tive brush was produced and a point of light on the positive ball, 

 while at 10 cms. and above there was a small negative brush as 

 before and no noticeable effect on the positive ball. 



The same experiments were next tried without the jars on the 

 machine and the results were much the same as before, except of 

 course that there was a diminution in the brightness of the spark. 



Modifications in the Phenomena of Expts. 1 and 2 by Varying 

 the Size of the Terminals. 



If the balls were of unequal sizes, the effects of methods 1 and 2 

 were remarkably modified. The small ball in each case had a 

 diameter of 2 cms. and the larger 4 cms. 



The small ball was first made positive, as is often the case with 

 many machines. All the effects of Expt. 1 were increased but 

 none of those of Expt. 2 could be obtained. 



The most interesting changes were produced when the smaller 

 ball was the negative terminal, when the effects varied with the 

 length of the gap ; at a distance 5 "2 cms. or less none of the effects 

 of Expt. 1 could be obtained, but if the spark was taken from the 

 positive side then at times a spark discharge took place ; when this 

 did not happen the negative brush appeared on the portion of the 

 negative terminal nearest the positive at the instant of taking the 

 spark. On gradually increasing the distance the negative brush 

 became fainter and fainter but could generally be seen in a 

 darkened room up to a length of 10 cms. At this point a spark 

 taken from the negative terminal produced discharge. No doubt 

 at this length the relative sizes of the two balls have not so great 

 an effect in determining the distribution of potential in various 

 parts of the field, so that the lines of force would be similar in 

 position to those existing under the conditions of Expt. 1, but 

 the increase was not so great, owing perhaps to the difficulty of 



