324 
NATURE 
[August 6, 1885 
ever since been considered as decisive in favour of the 
undulatory theory. 
We come now to our third question: Is radiant heat 
physically similar to radiant light? Here the difficulty is 
an instrumental one; the difficulty, in fact, of inventing 
something which shall do for dark heat what the eye can 
do for light. 
At a comparatively early period Sir John Leslie devised 
his differential thermometer, with which he was able to 
obtain valuable results, to be hereafter alluded to. In this 
instrument we have two bulbs, A and B, filled with air, 
and connected together by a bent tube (Fig. 2) the lower 
3B 
Fic. 2. 
portion of which is filled with some coloured liquid, which 
ought not to be volatile. Let us begin by supposing that 
both bulbs are of the same temperature, and that under 
these circumstances the air is at the same pressure in both. 
The line between C and D, the surfaces of the liquid, in 
the two tubes will consequently be horizontal. Now 
suppose that the bulb A is heated, its air pressure is in 
consequence increased, and hence the liquid will be pushed 
down at C andup at D. In like manner if B is heated 
the liquid will be pushed down at D and up at C, and the 
change may be roughly taken as proportional to the 
difference in temperature between the two bulbs, this 
difference being supposed to be small. If, however, both 
bulbs are heated simultaneously, and to the same extent, 
there will be no motion in the liquid, inasmuch as there 
will be no difference in pressure of the air of the two 
bulbs. 
In consequence of this mode of action the instrument 
has received the name of the differential thermometer ; 
Fic. 3 
3 
indeed, it is abundantly evident that what is measured is 
not the absolute temperature of A and B, but only the 
difference in temperature between the two. 
Delicate as this instrument might at first sight appear 
to be, it forms but a poor substitute for the human eye, 
and had it not been for a new discovery, we should not 
have been able to make much progress in our knowledge 
of dark heat. The discovery alluded to is that of Seebeck, 
| reference to this series. 
| one of its junctions as in the Figure. 
who found that in a circuit, composed of two metals 
soldered together, 2 current of electricity is produced when 
one of the junctions is heated, while the other is kept cool. 
If, however, both junctions be simultaneously heated to 
the same extent, no current is produced. 
Here, then, we have an instrument similar in principle 
to that of Leslie, or, in other words, a new species of 
differential thermometer, and we shall now show that this 
arrangement is capable of being made extremely delicate 
as a measurer of small differences of temperature. The 
existence of a current of electricity is easily known 
by the motion of a magnetized needle, which tends 
to place itself at right angles to the direction of the 
current. Suppose now we have a circuit (Fig. 3), in 
which C denotes copper and # bismuth, and that we heat 
We shall have, in 
consequence, a positive current following the direction of 
the arrow head, and the north pole of the needle will be 
pushed towards the observer as indicated in the Figure. 
When we make use of a magnet to measure a current we 
call our instrument a ga/vanometer. Our object, there- 
fore, in this arrangement, is clearly to get as large a 
current as possible out of a small temperature difference, 
and then to measure this current by means of a galvano- 
meter made as delicate as possible. 
In order to obtain as strong a current as possible we 
must make use of a considerable number of junctions, as 
in Fig. 4, only in practice these junctions are very close 
Hf £ AT 
\ Ree 
Fic. 4. 
together. Here the heating influence is applied to the 
upper junctions, while the lower ones are kept cool. 
Another point is to select two suitable metals for our 
junctions—that is to say, metals the heating of which 
shall produce a powerful current. This is done by con- 
sulting a thermo-electric list of metals ; in other words, a 
list such that the positive current shall go across the 
heated junction from the metal nearest the top to that 
nearest the bottom of the list. 
The following is a series of this nature : 
Bismuth. 
Nickel. Silver. 
Lead. Zinc. 
Tin. Iron. 
Copper. Antimony. 
Platinum. Tellurium. 
Now there is an important law which holds with 
If, for instance, we have a com- 
pound circuit, such as that in Fig. 5, connected with a 
galvanometer, we shall get the same current 77 one 
direction by heating through 1° C. the copper and 
tin junction, and also the tin and antimony junction, 
as we shall zz the opposite direction by heating the anti- 
mony and copper junction. In other words, the various 
metals in the above list are to be regarded as being at so 
many different levels, and the strength of the current 
depends upon this difference of level, and not at all upon 
the exact number of halting places we make use of in 
