the Spark-Length of the Wimshurst Machine. 63 



There seemed to be no necessity for the terminals to be of 

 the same size, nor for the spark-gaps to be equal, if one was 6 cms. 

 or more and the other only 2 cms. they synchronised just as 

 easily. 



When a board or plate of metal was placed between so that 

 the rays of one spark were shielded from the other, then the effect 

 was practically stopped, but even when the spark-gaps were not 

 so shielded from each other then one machine had to be charged 

 to very near sparking potential before the other discharged. In 

 the latter case the effect could be increased by bringing the 

 machines nearer together. 



(4) By holding a Rod of Insulating Substance next the 

 Positive Terminal. 



Humphreys has already dealt with this method (Phys. Review, 

 Vol. XI., No. 2). The machine was run continuously and the 

 spark-gap gradually increased until sparks ceased to pass, then on 

 bringing a glass rod near the positive pole a discharge took place. 

 The gap could in this way be increased 50 per cent. No effect 

 was produced by holding the rod at rest near the negative 

 terminal beyond a brush discharge between the glass and the 

 pole. On rare occasions if the rod was quickly moved past the 

 negative ball a discharge could be forced provided the machine 

 was near sparking potential at the time. 



Many different substances besides glass were tried and the 

 following were the results. 



Sealing-wax, ebonite, and many insulators all gave a result 

 similar to the glass rod. 



A stick of sealing-wax tipped with tinfoil, an insulated wire, 

 a glass rod with end white-hot, a candle-flame and tip of a 

 blowpipe flame produced discharge when passed near either 

 terminal; with the latter a slight shock was felt in the hand 

 holding the blowpipe. 



An air-blast either across the gap or along it produced no 

 effect whatever at either side. The wind was forced both from 



o 



o 



To Jar To Bellows 



To Jar 

 Fig. 2. 



positive to negative and in the opposite direction by having 

 a terminal as in Fig. 2. A was a glass tube holding the hollow 



