PROFESSOR J. H. POYNTING ON THE TRANSFER 
354 
Some of this energy which travels along the highest level surfaces will converge on 
the acid, and there be, at any rate, ultimately converted into heat. Some of it will 
move along those surfaces which crowd in between the acid and copper and there 
converge to supply the energy taken up by the escaping hydrogen. The rest spreads 
out to converge at last at different parts of the circuit, and there to be transformed 
into heat according to Joule’s law. 
It may be noticed that if the level surfaces be drawn with equal differences of 
potential, equal amounts of energy travel out per second between successive pairs of 
surfaces. For the amount transformed in the circuit in a length having a given differ¬ 
ence of potential V between its ends will be V x current, and therefore the amount 
transformed between each pair of surfaces drawn with the same potential difference 
will be the same. But since the current and the field are steady, the energy trans¬ 
formed will be equal to the energy moving out from the cell between the same surfaces 
—the energy never crossing level surfaces. This admits of a very easy direct proof, 
but the above seems quite sufficient. 
This result has a consequence, which though already well known, is worth mention¬ 
ing here. Let Y± be the difference of potential between the zinc and acid, V 0 that 
between the acid and copper. If i be the current, Y x i is the total energy travelling 
out per second from the zinc surface. Of this Y z i is absorbed at the copper surface, 
the rest, viz., {Y 1 — Y 2 )i, being transformed in the circuit. The fraction, therefore, of 
Y _y 
the whole energy sent out which is transformed in the circuit is a result analo- 
^ i 
gous to the expression for the amount of heat which can be transformed into work in 
a reversible heat engine. 
One or two interesting illustrations of this movement of energy may be mentioned 
here in connexion with the voltaic circuit. 
Suppose that we are sending a current through a submarine cable by a battery with, 
say, the zinc to earth, and suppose that the sheath is everywhere at zero potential. 
Then the wire will everywhere be at higher potential than the sheath, and the level 
surfaces will pass from the battery through the insulating material to the points where 
they cut the wire. The energy then which maintains the current, and which works 
the needle at the further end, travels through the insulating material, the core serving 
as a means to allow the energy to get in motion. 
Again, when the only effect in a circuit is the generation of heat, we have energy 
moving in upon the wire, there undergoing some sort of transformation, and then 
moving out again as heat or light. If Maxwell’s theory of light be true, it moves 
out again still as electric and magnetic energy, but with a definite velocity and inter¬ 
mittent in type. We have in the electric light, for instance, the curious result that 
energy moves in upon the arc or filament from the surrounding medium, there to be 
converted into a form which is sent out again, and which, though still the same in kind, 
is now able to affect our senses. 
