Electric Energy in Voltaic Circuits. 491 
be drawn are zinc and copper. Now thermo-chemical data 
lead me to conclude that the step of potential between clean 
zinc and air is 1°8 volt [the exact decimal not being insisted 
on except for purposes of illustration]; hence, if an equipo- 
tential line be drawn for every tenth part of a volt, 18 of these 
lines would closely invest clean zinc always, and form a 
boundary between it and the air. In the diagram (Pl. IV.) 
lines are only drawn for every fifth of a volt, and consequently 
9 of them envelop the zinc. 
As for copper, it is probable that the step of potential 
between it and air is *8 of a volt; and accordingly four lines 
invest the copper at every point in the diagram. They are 
not always the same four lines—that is of no consequence ; 
the essential thing to remember in drawing the diagram is 
to keep the number constant at every point for the same 
metal. Such a metal as gold or platinum may be nearly free 
from lines. 
It is needless to say that the lines have to be drawn with 
spaces between them for distinctness, but that in reality the 
step of potential is of the most sudden description, and all 
investing lines ought to lie close to the mathematical surface 
of the metal. 
Now, since nine lines coat the zinc, and only four the 
copper, it is manifest that five lines must leave the metallic 
surface at the junction, and must spread out into the air. 
These five lines correspond to the one volt* Volta effect 
observed near a junction of clean zinc and copper, and which 
has been called a difference of potential between zinc and 
copper. 
Of the five lines thus starting to spread out from the junc- 
tion, some make for points on the external circuit, and others 
go through the internal circuit. Those which go to the ex- 
ternal circuit can by no means go through it, or into it, for the 
potential of the copper differs -by °8 volt from that of the air ; 
but they can go round it, and leave it on the other side just 
as if they had gone through it. Moreover, a line does go 
through, or into, the copper at the very point where the air- 
line abuts against it, viz. the line of potential °8 volt lower. 
So the transfer of energy into the conductor of the external 
* Different observers give different values for this number; from ‘5, 
Kohlrausch, to between ‘76 and ‘89, Ayrton and Perry and Clifton, up to 
‘92, Pellat; but since tarnish or dirt on the zinc surface will certainly 
lower it, and it is difficult to suppose any zinc surface absolutely clean, I 
have not thought it necessary to modify the theoretical value of one 
volt, which at any rate serves well enough for illustration, though it may 
turn out to be about ten per cent. too high. 
