160 
MR. C. V. BOYS OY THE RADIO-MICROMETER 
It may, perhaps, be worth mentioning that 1 was led to attempt to make some 
improvement in the thermopile in consequence of the extraordinary sensibility 
which Professor Langley had given to the bolometer, because it seemed incredible 
that, with so small a temperature coefficient of resistance as metals possess, an instru¬ 
ment depending on change of resistance should compare favourably with one equally 
well carried out, in which the comparatively large electromotive force of a thermo¬ 
electric junction is made use of. 
The first object obviously was to reduce the mass of the exposed part of the pile. 
I was, therefore, naturally led to the use of fine wires or thin plates connected with 
a galvanometer in the usual way, but all the forms which such an instrument might 
take seemed to promise very little after the idea of the suspended circuit occurred to 
me. I, therefore, put them on one side, and devoted myself to the perfection of the 
instrument which forms the subject of the present paper. I notice, however, that 
there is an account of an instrument of such a kind in the January Number of the 
‘ Philosophical Magazine’ of this year (1888). 
The considerations which led to the use of the general form of circuit, ix., one 
composed of a thin flat bar of antimony and bismuth, having its ends connected by a 
thin copper wire, which forms the remaining sides of a square or rectangle, are very 
simple. 
A pair of metals must be chosen which have a high thermo-electric power, and 
which are not to any great extent magnetic, and which can be made exceedingly 
thin. Of ordinary metals antimony and bismuth so excel others in thermo-electric 
power that, unless they fail in other respects, they will be the best for the purpose. 
The diamagnetism is but a small disadvantage, and this may be overcome. The 
great density of the metals is objectionable ; further, the difficulty of making the 
circuit increases as these metals are made thinner. The low conductivity for elec¬ 
tricity of these metals is also a disadvantage, but this is balanced by their corre¬ 
spondingly low conductivity for heat. The disadvantages seem to be more than 
outweighed by the great thermo-electric power of the combination, if no attempt is 
made to complete the circuit with these metals only, but if copper is used for this 
purpose. In this way, the strongly diamagnetic metal is kept out of the intense part 
of the field; the high conductivity compared to its mass of copper, in wdiich respect 
aluminium only is superior (but this cannot be soldered), is made use of to convey the 
current from one end of the bar to the other, round a circuit which may be of 
sufficient extent to enclose a large area in the magnetic field. Thus, the copper part 
of the circuit may have less weight and less resistance than it would have if made of 
antimony and bismuth. 
As to the form of this hoop of copper, mechanical considerations determine that it 
shall be rectangular, for the pole pieces and central core could not conveniently be 
made to suit a circuit of other form; otherwise it wmuld appear that a cmcular or 
elliptic form would be preferable. 
