ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING FIELD. 289 
merely as a decay of electric induction without movement inwards of fresh induction 
tubes, and therefore without the formation of magnetic induction. 
I have discussed the case of discharge of a condenser at some length, as we can 
here realise more easily what goes on at the source of energy. The results obtained 
suggest that a similar action occurs at the source of energy or seat of the electromotive 
force in other cases where we do not know the distribution of induction, and are 
obliged to guess at the action. 
A circuit containing a voltaic cell. 
We may pass on from the discharge of a condenser to the consideration of the 
current in a circuit containing a voltaic cell. The chemical theory of the cell will be 
here adopted—in fact, the hypothesis I am endeavouring to set forth has no meaning 
on the voltaic metal-contact theory. 
Let us suppose the cell to consist of zinc and copper plates, a vessel of dilute 
sulphuric acid, and copper wires attached to each plate which on junction complete the 
circuit. For simplicity I shall disregard the effect of the air and suppose that it is a 
neutral gas causing no induction. 
We shall begin by supposing the circuit open. Then we know that on immersion 
there will be temporary currents in the wires, the quantities of these currents depend¬ 
ing on the electrostatic capacity of the system composed of the wires. The currents 
last till the wires have received charges such that they are, say at difference of 
potential V. If the terminals are connected to a condenser the temporary currents 
may be easily detected by a galvanometer in the circuit. They are in no way to be 
distinguished in kind from the permanent current which will be established when the 
circuit is complete, except that they are of short duration and in general very small. 
There is no reason then to suppose that the action in the cell is different from that 
which takes place when the current is permanent, and I think we may safely assume 
that Faraday’s law of electrolysis holds according to which the quantity of electricity 
flowing along either wire is proportional to the quantity of chemical action—or, in the 
form appropriate here, the number of tubes of induction produced is proportional to 
the quantity of chemical action. 
Let Q be the total quantity of electricity upon the positive terminal; then is 
the total energy thrown out into the dielectric. 
Let z be the quantity of zinc consumed per unit of electricity, then Qz is the total 
quantity consumed in the charging of the terminals. Let E be the energy set free by 
each quantity 2 of zinc consumed, after all actions in the cell have been provided for. 
E then is the E.M.F. which the cell will have on the closure of the circuit, as long as 
the chemical actions remain the same, for z corresponds to the passage of a unit of 
electricity or the production of one tube, and we know that the energy set free by 
C units is CE. 
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