ON ELECTRICAL DISCHARGES THROUGH RAREFIED GASES. 
177 
III.— The effect produced on a sensitive discharge hg the approach of a conductor, is 
directly due to the relief given by its presence to the instantaneous electric 
tension within and around the tube, caused by the individual discharges in 
their passage through the tube. 
In the case of the sensitive or interrupted discharge we have seen that there are 
discrete pulses of electricity passing between the terminals. It is, therefore, not an 
improbable supposition that the pulse of positive electricity leaving the positive 
terminal in the form of free electricity, and consequently exercising electric induction 
in every direction round it, may cause that electric tension or (as Professor Maxwell 
would call it) displacement, which, so far as we know, is the universal accompaniment 
of free electricity. Such induction would cause an electric tension upon the outer 
surface of the glass and in the space beyond ; but, inasmuch as the glass is a non¬ 
conductor, it could not cause any redistribution of the electricity within it or on its 
surface. It is somewhat difficult to ascertain the effect on the space around the tube, 
occupied as it is with the mobile and rapidly-moving particles of air; but there can be 
little doubt that the induction from the discharges would only occasion redistribution 
of the electricity in this part of the field to a very slight extent. If, however, we 
place a conductor on or near the glass, the induction can readily occasion a redistribution 
of electricity in the conductor. The state of the electric field in the neighbourhood of 
the conductor would then be different to that at any other part of the tube ; and this 
would in its turn react upon the discharge or upon the gaseous matter which exists 
within the tube. We shall now show that the observed effect is due to such a 
redistribution of electricity in the conductor; and shall subsequently proceed to 
examine the cause of this redistribution and its modus operandi in producing the 
observed effect. 
In order to demonstrate that the phenomena of sensitiveness are primarily due to 
a redistribution of electricity in the conductor caused by alterations in the electric 
condition of the interior of the tube, it is only necessary to observe that a non¬ 
conductor, however highly charged, does not affect the sensitive discharge. Nor will 
a conductor of small size, in contact with the tube (such as a piece of tinfoil), affect 
the discharge so long as it is insulated. But if the tinfoil be connected to earth, 
or to a distant conducting body, it at once produces a marked effect on the sensitive 
discharge. 
A consideration of these experimental facts leads to the following conclusions: (1) 
that the effect is due to a redistribution of electricity in the conductor; and (2) that 
such effect is periodic. If a continuous electric state in the external body were the 
necessary condition, the observed effect could be produced by charged non-conductors. 
But as this is not the case, we must look to the facility of change in electric state 
mdccclxxix. 2 A 
