180 
MESSRS. W. SPOTTISWOODE AND J. FLETCHER MOULTON 
The arrangement in which the tinfoil is connected to earth is that which affords the 
most complete relief to the periodic changes of electric tension produced by the periodic 
pulsations of electricity through the tube. And, considering that the earth may be 
regarded as an infinite conducting body of potential zero, it seems justifiable to use the 
term complete relief in connexion with its effect. But in order to render the relief 
complete, it is not necessary that the potential of the relieving conductor should be 
zero; it is the relief to the periodic alternations of electric tension which produces the 
effect. If a Leyden jar be connected with the tinfoil, its effect on the sensitive 
discharge is the same whether it be charged or not. In short, any conductor of 
sufficient size connected with the tinfoil will produce the same effect as the earth, at 
whatever potential it stands. 
It is so important to establish clearly that the effect of a conductor is independent 
of the potential at which it stands, and is due only to the facility for redistribution of 
electricity which it affords, that we shall here refer to an experiment which very 
clearly demonstrates such to be the case. Let a Holtz machine be taken as the source 
of the current, an d an air-spark be interposed at some part of the current outside the 
tube, say, for exa mple, in the part between the tube and the positive terminal of the 
machine, and let a ring of tinfoil be placed round the tube and connected with the 
earth and the relief-effect duly noted. It will be found that no effect is produced 
either upon the current'" or the relief-effect by connecting either of the terminals of 
the machine to earth. That no effect should be produced upon the current is not 
surprising, as the whole effect, like all other electro-dynamic effects, must be due to 
the difference of the potentials of the two terminals and not to their absolute values. 
But it is very remarkable that the relief-effect is the same in both cases, inasmuch as 
in one case the tinfoil is of the same potential as the positive terminal of the machine, 
which is of higher potential than any other part of the whole circuit (including the 
tube), and in the other case it is of the same potential as the negative terminal of the 
machine, which is necessarily of lower potential than any other portion of the circuit. 
No stronger proof could, we think, be given that the effect is completely independent 
of the potential of the relieving system. 
From this follows a remarkable result, the accuracy of which is, however, fully 
* So far as the current is concerned, the actual experiment when made by the authors of this paper 
would not support this statement, owing to a peculiarity in the source of electricity then used. This 
source was a large Holtz machine, and it was discovered that there was a large excess of positive 
electricity in the currents produced by it, so that when the terminals were metallically connected, sparks 
of a considerable length could be obtained from them. The obvious consequence was that the current 
through the tube was stronger when the negative terminal was connected to earth than when the positive 
terminal was so connected. But when allowance was made for this instrumental peculiarity the experiment 
fully supported the above statement. The peculiarity itself was doubtless due to an escape of negative 
electricity into the air within the machine, 
