
TO A THERMO-ELECTRIC DIAGRAM. 133 
“ When the quantities to be found are very small, as for instance in the case 
Ag — Cu, we cannot expect to get a good approximation by introducing a third 
metal. In fact, introducing Fe we find indirectly 0:00147 — 0:00151 = — 0:00004, 
while the direct determination gives — 0:00006. 
«“ Aoain with Zn and Cu, indirectly we get 
— 000042 and — 144°C. 
Directly — 0:00048 and — 146°C. 
“Several of the other groups give results as closely agreeing with one 
another as these, others are considerably out. 
“The numerical determinations above are founded entirely on a series of 
experiments made for me by Messrs J. Murray and R. M. Morrison. Mr W. 
DuvrHAM is at present engaged in determining the electro-motive force of con- 
tact of wires of the same metal at different temperatures, with the view of 
inquiring into its relation to ordinary thermo-electric phenomena which appears 
to be suggested by some of the formule above given.” 
Mr Duruaw’s results were published in the Proc. R.S.L. (June 17th 1872), 
and showed that in the case of platinum, the only metal he examined, the 
integral deflection of a somewhat massive galvanometer needle is independent 
of the absolute temperature of either wire, and proportional simply to the 
difference of their temperatures. This was the result I had expected from the 
formule given above (p. 130) ; for if 
ka = hy t} 
we have 
ie = eo 5 
but consistently with these we may have 
(Ka ri ky) Tas = Ty 
a finite quantity. Hence 
E = Jr (t—i). 
Various other communications on the subject were made by me to the 
Society, and published in the Proceedings ; but of these I need quote only the 
following, of date June 3d, 1872, as it shows a novel difficulty which I met 
with, and which prevented me from publishing earlier an attempt at construct- 
ing a thermo-electric diagram :— 
“Having lately obtained from Messrs JoHnson and MaTTHEY some wires 
of platinum, and of alloys of platinum and iridium, I formed them into circuits 
_ with iron wire of commerce ; and noticed that with all, excepting what is 
called ‘ soft’ platinum, there is more than one neutral point situated below the 
temperature of low white heat, and that at higher temperatures other neutral 
points occur. This observation is, in itself, highly interesting ; but my first 
VOL. XXVII. PART I. 2M 
