352 Prof. Oliver Lodge on the Seat of the 
Volta force be equal to each individual true force or not. See 
section 7. 
xvill. Wherever a current flows across a seat of E.M.F., 
there it must gain or lose energy at a rate numerically equal 
to the H.M.F’. multiplied by the strength of the current*. 
Development of the above and special application to Metals. 
xix. A metal is not at the potential of the air touching it, 
but is always slightly below that potential by an amount 
roughly proportional to its heat of combustion, and calculable, 
at any rate approximately, from it. For instance, clean zine 
is probably about 1:8 volt below the air, copper about °8 volt 
below, and so on. If an ordinary oxidizing medium be sub- 
stituted for “air’’ in the above statement it makes but little 
difference. 
xx. Two metals put into contact reduce each other instantly 
to practically the same potential; and consequently the most 
oxidizable one receives from the other a positive charge, the 
effect of which can be observed electrostatically. 
xxi. There is a slight true contact-force at the junction of 
two metals which prevents their reduction to exactly the same 
potential; but the outstanding difference is small, and varies 
with temperature. It can be measured thermoelectrically by 
the Peltier effect, but in no other known way. It is pro- 
bably entirely independent of surrounding media, metallic or 
otherwise}. 
xxii. If two metals are in contact, the potential of the 
medium surrounding them is no longer uniform: if a 
dielectric it is in a state of strain, if an electrolyte it conveys 
a current. 
xxiii. In the former case the major part of the total dif- 
ference of potential is related closely to the difference of 
the potential energies of combination, and is approximately 
calculable therefrom. In the latter case the total H.M.F. 
is calculable accurately from the energy of the chemical 
* A current gains energy at any junction at which heat is absorbed, or 
chemical combination permitted, or any other form of energy destroyed, 
by the passage of the current. The current gains the energy which has 
in the other form disappeared. 
A current loses energy at a point where it causes other forms of energy 
to make their appearance; e. g. generation of heat, decomposition of che- 
mical compounds, &c. 
1 To distinguish between Peltier-force and Volta-force henceforward 
it will be best to write Bi/Sb or Zn/Cu for the former, and Zn/Air/Cu or 
Fe /Water / Pt for the latter. The force electroscopically observed is 
Air/Zn/Cu/Air, but this involves both; the right way of denoting the 
Volta effect pure and simple is Zn/Air/Cu. 
