d44 Prof. Oliver Lodge on the Seat of the 
The same chemical action goes on in each, zinc is dissolved 
at one end and hydrogen liberated at the other: how, then, can 
the H.M.F. be different if it is calculable from the chemical 
reactions ?* 
If we picture to ourselves the actual forces in action we 
shall get a kind of answer indicated to us. In a zinc-iron 
cell the E.M.F. is due to the zine pulling at oxygen harder 
than the iron does ; but, since the iron does pull too, with no 
inconsiderable strength, the balance of force is not so great 
as if the iron were replaced by copper, which pulls less, or by 
platinum, which barely pulls at all until it is coated and 
alloyed with hydrogen. 
This answer cannot be considered as complete, and in order 
to complete it consider a more precise experiment. 
Arrange a series of common dilute acid voltameters, with 
their plates respectively, zinc zinc, zinc iron, zinc copper, and 
zinc platinum. Pass one current through the series, from zine 
to the other metal, and measure the differences of potential 
between the plates in each cell. Now the same chemical 
action is going on in each. In each, zinc is dissolved at one 
side and hydrogen evolved at the other—the only difference 
being that it is liberated from surfaces of zinc, iron, copper, 
platinum, in the four cases. What is to prevent the H.M.F. 
between the terminals of each voltameter from being the 
same? But itis not the same (pace Prof. Exner): the zinc- 
zinc cell shows the greatest difference of potential between its 
terminals, the zinc-iron less; and the zinc-platinum may 
easily show a reverse difference because it helps the current on 
instead of hindering it. It will be understood that the precise 
behaviour of the cells is determined by the intensity of the 
current (7. e. current per area)—if it is weak, even the zinc- 
iron cell may help it on, but the zinc-platinum will help it on 
most: if it is very strong, even the zinc-platinum will retard 
it, but the others will retard it more, and the zinc-zine most. 
Now why is all this? ‘Take the difference between the 
heats of formation of Zn, SO, and of H,, SQ,, at the comma, 
and you will have the total energy assimilated by the current 
* Professor Exner cuts this knot in characteristic fashion by asserting 
roundly that the E.M.F. of all such cells is the same, and that it matters 
nothing what metal is opposed to the zinc of a cell so long as it does not 
alter the chemical action going on. He further asserts that all batteries 
are non-polarizable and quite constant, as soon as they have got rid of 
dissolved air and before sulphate of zinc has accumulated. He verifies 
these extraordinary statements, to three significant figures, by straight- 
forward experiment. See his paper “On Inconstant Voltaic Batteries,” 
cited above. 
