On the "Radio-micrometer." 



189 



March 24, 1887. 



Professor STOKES, D.C.L., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



Major A. W. Baird, R.E. (elected June 4, 1885), was admitted into 

 the Society. 



The following Papers were read : — 



I. " Preliminary Note on the ' Radio-micrometer a New 

 Instrument for Measuring the Most Feeble Radiation." By 

 C. V. Boys, Demonstrator of Physics at the Science 

 Schools, South Kensington. Communicated by Professor 

 A. W. Rucker, F.R.S. Received February 24, 1887. 



Till lately the thermopile and galvanometer have afforded the 

 most delicate means of detecting and measuring radiant energy. This 

 combination has been surpassed by Professor Langley, who has made 

 use of the increased resistance of metallic wires or the diminished 

 resistance of a carbon filament when warmed. 



It seems difficult to believe, in consequence of the very small change 

 of resistance which even iron undergoes when slightly warmed, that 

 this is the best principle to make use of in designing the most sensi- 

 tive possible instrument. I felt that if an instrument depending on 

 thermo-electric force could be made as beautifully as Professor 

 Langley's bolometer, a better result ought to be obtained. The one 

 point in which the bolometer has a great advantage is the small mass 

 of the part to be heated, whereas a thermopile, delicate as the bars 

 may be, has a mass so enormous that both the rate of heating and the 

 ultimate rise of temperature must be small in comparison. 



A thermopile with bars as thin as the wires of the bolometer cannot 

 be made with antimony and bismuth, and yet such a construction 

 would be required before the thermo-electric force could be utilised to 

 the same extent that the change of resistance is in the bolometer. 



If the conducting wires had no resistance, no advantage would be 

 gained by having more than one junction, provided that the galvano- 

 meter were properly proportioned. If it were not for the resistance 

 and torsion of the stretched wires, the moving coil galvanometer of 



