ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD. 287 
Now let the two plates be connected by a wire. Discharge takes place, and we are 
fairly justified, from the heat in the wire and the transient magnetic effects, in saying 
that a current has been in the wire from the positive to the negative plate, or the 
wire was for the time being in the same relation to the surrounding medium as the 
wire in the case just considered, the condition of affairs, however, not being steady. 
Let us suppose the wire to have a very great resistance, in order that, at least in 
imagination, we may lengthen out the time of discharge. On the ordinary current 
theory, combined with Maxwell’s “displacement” theory, the medium between the 
plates has returned from the strained condition, denoted by “ displacement ” from the 
positive to the negative plate, causing displacement through the plates and along the 
wire, the displacement being in the same direction all round the circuit. This is 
generally, I think, supposed to take place by the recovery of the medium between 
the plates causing displacement in the metal immediately in front of it, the displace¬ 
ment being analogous to the forcing of water along a pipe corresponding to the plates 
and wire, by the recovery from strain of some substance placed in a chamber 
corresponding to the space between the plates. 
According to the hypothesis here advanced we must suppose the lessening of the 
induction between the plates—induction being used with the same physical meaning 
as Maxwell’s displacement—to take place by the divergence outwards of the 
induction tubes. We may picture them as taking up the positions of successive lines 
of induction further and further away from the space between the plates, their ends 
always remaining on the plates. They finally converge on the wire, and are then 
broken up and their energy dissipated as heat. At the same time some of the energy 
becomes magnetic, this occurring as the difference of potential between the plates 
lowers, so that the tubes contain fewer unit cells. 
The magnetic energy will be contained in ring-shaped tubes which will expand from 
between the plates and then contract upon some other part of the circuit. To 
illustrate the movement of the electric induction tubes let us suppose them to be 
represented by elastic strings stretched between the two plates. Then the motion of 
the tubes outwards would be roughly represented by pulling the elastic strings 
outwards and doubling them back close against the wire, their ends being still 
attached to the plates. It is evident that if any ring surround the wire each of the 
strings must break through it in order to reach the wire. Hence the total number of 
strings cutting any ring surrounding the wire is the same wherever the ring be placed. 
Similarly the total number of tubes of electric induction cutting any curve encircling 
the wire is the same, and therefore the line integral of the magnetic intensity round 
the curve integrated throughout the time of discharge is the same, or the total magnetic 
effect is the same at all parts of the circuit. It is not necessary to suppose that a 
tube enters the wire at the same moment throughout its whole length; indeed, the 
experiments of Wheatstone on the so-called velocity of electricity prove clearly that 
2 p 2 
