60 Mr Palmer, Some Methods of Increasing 



Some Methods of Increasing the Spark-Length of the Wims- 

 hurst Machine. By B. J. Palmer, B.Sc. (Communicated by 

 Professor J. J. Thomson.) 



[Read January 30, 1905.] 



Some interesting experiments have recently been published 

 by Humphreys (Physical Review, Vol. x., No. 5) showing how 

 the normal spark-length of a Wimshurst can be greatly increased 

 by taking a spark from the negative terminal. Professor J. J. 

 Thomson suggested I should endeavour to measure this increment 

 and so find out its cause. My experiments in this direction were 

 entirely unsatisfactory, but incidentally I noticed several other 

 ways in which discharge could be forced, and the following paper 

 deals with these. 



Many of the phenomena about to be described must have been 

 noticed before by different experimenters, but I do not know of 

 any research other than Humphreys' dealing with the subject, 

 so that they will be new, in part at least, to many. 



The magnitude of the lengthening depends on several things, 

 the size, shape, and degree of polish of the terminals between 

 which the discharge takes place, the speed with which the 

 machine is run, the atmospheric conditions, and the size of the 

 Ley den-jars on each terminal. 



Unless otherwise stated the following experiments were 

 performed with a two-plate Wimshurst, with plates of glass 

 40 cms. diameter, brass sectors and terminals of equal size, 25 cms. 

 in diameter, and with jars on the machine. 



The following were the methods : — 



(1) By taking a Spark from the Negative Terminal. 



The machine was kept running continuously with uniform 

 speed and the terminals gradually separated until sparks ceased 

 to pass ; this took place when the gap was about 10 cms. across. 

 On placing the finger near any part of the negative terminal so 

 that a small spark passed to the knuckle, then a bright discharge 

 took place between the balls ; the distance between them was 

 further increased and the discharge could in this way be forced 

 through a distance of 15 cms. If the gap was still greater, a 

 bright discharge no longer passed but a brush discharge took 

 place across it. At a distance of 16 cms. and upwards a brush 

 appeared on that point of the positive ball nearest the negative 

 terminal, and this brush either lost itself in the air between the 



