14 Miss Martin, Expansion 'produced [Nov. 11, 



doubtedly accompanies it. On the other hand no luminous dis- 

 charge ever occurs without a corresponding movement of the 

 gauge, and it is only when the sudden change of potential of 

 the two tubes is sufficient to cause a visible discharge in the 

 space between them that any alteration in volume of the gas 

 therein contained is to be observed. 



Of course it might be urged that though a luminous discharge 

 is always accompanied by an increase of volume in the gas the 

 reverse may not be the case, but that sudden changes of potential 

 too weak to give any luminous effect may produce a change in 

 volume which the form of gauge used is not sufficiently sensitive 

 to note. 



I do not think this is the case. In the first place the gauge 

 is so sensitive that if two fingers be laid against the top of the 

 outer tube the acid in the gauge below begins to move in from 

 1" to 1"'5 and in 17" moves through a distance of 20 mm. There 

 is however a much more conclusive proof that the gauge takes 

 account of every change of volume and that every such change is 

 accompanied by a luminous discharge through the gas. If the 

 distance between the terminals of the Wimshurst be so arranged 

 that the sudden fall of potential caused by the passing of the 

 spark is sufficiently great to cause luminosity in the space be- 

 tween the two tubes upon every discharge there is always a move- 

 ment of the acid in the gauge ; but when the fall of potential does 

 not give rise to a luminous discharge there is never the slightest 

 movement of the acid. 



When however the distance between the Wimshurst terminals 

 is such that the sudden discharge causes sometimes a faint 

 luminosity through the gas and sometimes none, the gauge some- 

 times moves and sometimes does not. It would perhaps be more 

 satisfactory to have a second observer in order to be perfectly 

 certain that the two effects synchronize, but I do not think there 

 can be any real doubt upon the matter. For it is possible so 

 to arrange things that the majority of discharges of the condenser 

 shall be accompanied by a luminous induction discharge, in which 

 case the acid generally but not always moves ; if now the Wims- 

 hurst terminals be caused to approach very slightly nearer to- 

 gether so that the maximum charge possible upon the condenser 

 plates is very slightly decreased, a smaller percentage of the 

 discharge is accompanied by the luminous appearance in the 

 tube, and the acid in the gauge moves a correspondingly fewer 

 number of times. 



This is the result of several hundreds of observations. It 

 is of course only within extremely narrow limits that this un- 

 certainty as to whether the sudden discharge will or will not 

 cause the effects under consideration is observable. It seems 



