290 
PROFESSOR J. H. POYNTING ON ELECTRIC CURRENT AND THE 
Now while the charges are gathering and while the potential difference of the 
terminals is gradually increasing, the energy required to add equal increments of 
charge will also increase, and the charging will cease when the amount of energy 
given up by a given amount of chemical action in the cell is equal to the amount 
required to add the corresponding charge to the terminals. For to suppose the 
action to go beyond this is to suppose that the energy thrown out into the space 
between the terminals is greater than that yielded by the battery. 
Let clQ be the last quantity of charge added to the terminals. This requires 
energy VdQ. 
The corresponding quantity of zinc consumed is zdQ, giving up energy EcZQ. 
The condition of equilibrium is that 
VcZQ=EcZQ 
or 
V=E 
which agrees with the result of experiment that the difference of potential of the 
terminals in open circuit is equal to the E.M.F. of the cell immediately after closure. 
It may be noticed that the total quantity of energy extracted from the battery is 
QE=QV 
while the electric energy left in the medium is 
QV 
2 
or half the energy has been converted into heat in the wires. 
We will now consider the distribution of level surfaces in the field while the circuit 
is still open. There will be V — 1 surfaces between the terminals dividing each tube 
into Y cells. None of these will cut the homogeneous parts of the circuit, since the 
whole of each of these must be at one and the same potential. 
They can only cut the circuit by passing through the regions where there is contact 
of dissimilar bodies. We will neglect the contact of the zinc and copper, as the 
difference of potential there is insignificant compared with that at the two surfaces, 
zinc-acid and copper-acid. 
Now we know that the energy of the cell is put out at the zinc-acid contact, but 
the amount is greater than that obtained from a consideration of the E.M.F. of the 
cell, for some energy is absorbed again, probably, at the copper-acid contact in the 
evolution of hydrogen. There is probably, then, induction between the acid and the 
zinc, and between the acid and the copper, these resembling the spaces between the 
plates of two condensers, the acid being at a higher potential than either. But if 
a given amount of induction disappears from the zinc-acid contact and appears at the 
terminals, more energy is lost at the former than appears at the latter. Hence all the 
cells have not been transferred from one to the other, or the difference of potential 
