Electromotive Forces in the Voltaic Cell. Shs, 
volt, and is such as to drive the current from zinc to copper 
across the cooler junction; at least this is true above —60° or 
—80°*. 
Hitherto we have supposed. the circuit to be all at one 
temperature ; but if different parts are at different tempera- 
tures, we shall have to use a yet further series, viz. a Thomson 
series, for the H.M.I. acting in any one substance with a 
difference of temperature between its ends, or the force acting 
at a junction of two pieces of the same metal at different 
temperatures. This series can be deduced from the preceding, 
using only the coefficient of ¢?, and multiplying it by the 
difference of the squares of the absolute temperatures of the 
two ends of the piece of metal. Such a series then stands 
thus :— 
Thomson Series, or H.M.F’. in a metal whose ends differ 
in temperature. (Microvolts.) 
Pom ee. 0487 (G8) {274 3G Th) 
German silver . “0512 (4, -&){274+3(4,+%)} 
Mae s . . "024 (1-8) {2744+ tee) t 
and so on. 
Whether a series of this sort can be made to include any 
non-metallic conductors also, has not yet been discovered. 
M. Bouty’s experiments provisionally indicate the very inter- 
esting fact that Sir W. Thomson’s general thermodynamic 
laws of the thermoelectric circuit apply perfectly to circuits 
which include some electrolytes as well as metals. 
Now the meaning of statement No. xxiy. is as follows: 
Regard zinc and copper in contact as a circuit completed by 
air or by water, as the case may be, and let the temperature 
be uniform, and say 10°; to reckon up the total H.M.F. we 
must look in the proper Volta series for Zn/air (or Zn/water), 
which we find 1°8 say; for Zn/Cu, which we don’t find, or 
find zero ; for Cu/air, which we find ‘8. Then we must look 
in the 10° Peltier series for Zn/air or Zn/water, which at 
present we shall not find there for want of data (possibly we 
have no right to put them there if we had data) ; for Zn/Cu, 
* It is always easy to tell from thermoelectric data which way the 
force acts at a junction; but it is not always the same way as the current 
flows, by any means. A current, excited by differences of temperature in 
a simple metallic circuit, may be urged against the force at both junctions. 
This is the case, for instance, in a copper-iron circuit with one junction 
above 275° and the other below it by a greater amount. It is customary 
to say that the current flows across a hot junction from the metal of 
higher-to the metal of lower thermoelectric value: this is not necessarily 
true. The safe statement is to say that the electromotive force acts. from 
high to low thermoelectric value, at either junction. 
