= 
. 
F 
Contrary Aexure, and then a minimum; the maximum 
and minimum being the stationary points corresponding 
to the two occasions dn which the expenditure equals the 
income. The maximum and minimum will obviously be 
farther apart, and smaller, the larger is the expenditure 
coiipaben with the minimum income. 
_ The latter part of these statements is well exhibited by 
the behaviour of circuits of iron, and various alloys of 
platinum with Iridium, Nickel, and Copper. 
[Some of these, involving two, and in on? case three, neutral 
points, were shown. ] 
In each of these cases there are obviously two neutral 
points, at least. Now suppose the two junctions raised to 
the temperatures of these two neutral points respectively, 
and we have a thermo-electric current maintained ev//re/y 
by the specific heat of electricity, as there is obviously 
neither absorption nor evolution of heat at either junction. 
Still further, suppose (as is very zedr7y the case with one of 
the alloys I have just used) that the specific heat of elec- 
tricity is x77 in the metal associated with iron, and we 
have the very remarkable fact of a current maintained in 
a circuit, without absorption or evolution of heat at either 
junction or in one of the metals, but with evolution of 
heat in one part of the sccond metal and absorption in 
another part. This suggests immediately the idea that iron 
becomes, as it were, a different metal on being raised above 
a certain temperature. This may possibly have some 
connection with the Ferricura and Ferrosum of the 
chemists ; with the change of magnetic properties of 
iron, and of its electric resistance, at high temperatures. 
Dr. Russell has kindly enabled me to verify these 
properties in a specimien of pure iron prepared by 
Matthiessen. I find similar effects with Nickel at a much 
lower temperature. The method of control which I em- 
loyed to satisfy myself that these peculiarities are due to 
iron and not to the platinum alloys, requires a little ex- 
planation. It depends upon the fact that by the 
help of two métals made into a double are (wires of 
the two being stretched side by side, without contact 
except at the ends) we can explore any portion of the 
field between the lines for these two metals by simply 
altering the ratio of the resistances in the two parts 
of the double arc, Such a_ complex arrangement 
gives a line  passitg through the intersection of 
thé lines of the two constituents, atid depending for 
its position on their relative resistances. I shall not, 
at this stage of my lecture, trouble you with the formula 
which gives the line for the double are in terms of the 
resistances of the two metals and their lines, but simply 
show the experiments with the help of a gold and a 
palladium wire, the one having the specific heat of elec- 
tricity positive, the other riegative; while their neutral 
point is considerably below the temperature of the room: 
Between their lines is included the peculiar portion of the 
iron line, and by making shots at it, as it were, in various 
directions from the neutral point of gold and palladium, 
we shall be able to study its bearings. 
[Several of these experiments were showy till finally the gold 
wire was melted.] 
I have heté Wires of iron, gold, and palladium, bound 
together at oné énd, which is to be the hot junction. One 
eta of the galvanometer coil is connected With the free 
end of thé ifon wire, the other slides along 4 long coppet 
wiré which Conriects the free ends of thé gold and palla- 
diffi wires. By sliding it towards either I diminish the 
resistatice of that branch of the double arc and iticrease 
that in the 6ther—.e, I give that brarich of thé double 
arc thé greater importance in the combination. 
Throwing the greater part of the resistance into the 
palladium branch, I find a neutral poitt at a moderate 
ie ature; btit I caftiot reach 4 Secotid without pouee 
d. Throw tote resistance into old, the fit 
Soiat seours at & pear tenet tha Before: 
NATURE 
123 
but a second is attainable. By still further increasing 
the resistance in the gold the two neutral points gradually 
approach one another, one rising in temperature the other 
descending, until at last we reach a maximum-minimum, 
the result of the confluence of the two points. The line 
for the double arc is now such as to ‘ouch the iron line. 
Still further increase the resistance of the gold, and we 
find a mere poiat ef inflexion, the galvanometer indica- 
tions having constantly rzsex, though at a retarded and 
then accelerated rate, during the heating of the junction. 
Two of the platinum alloys which I employed with iron 
seem to give lines almost exactly parallel to thé léad line 
—i.¢. in them the speciiic heat of electricity is practically 
nil. When a circuit is formed of these alloys the current 
therefore depends upon the Peltier effects at the junctions 
alone, and is sensibly proportional ti the difference of 
their absolute temperatures, thus furnishing a very 
convenient thermometer for the approximate estima- 
tiori of high temperatiires. [ am at préSent engaged in 
drawing the thermo-electric diagram in terms of tempera- 
tures as given by this coibination, and thé reduction to 
absolute temperattires will finally be effected by a com- 
parison of this temporary but very cohveitiént standard 
with an air-thermometer. P. G. Tair 
Nore.—The following rude sketch of a part of the thermo- 
electric diagram will perhaps render some of the pre- 
ceding remarks more intelligible. It is drawn to 
illustrate qualitative effects alone, 
; ‘ MY ocd «a 
The following diagram exhibits the amount of the 
Thomson and Peltier effects, and of the electromotive 
force, in a copper-iron circuit, the temperatures of both 
Junctions being under that of the neutral point, 
on 
el 
Peltier efféct at cold junction = Atea A Dda (heating) 
” ij. . HOE ” = 35 BCcéd (cooling) 
Thomson effect in capper ZR Deve 55 
Pas Fa { ton So “ i 
Electrombtive atte a Reep ” 
, eT. 
