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V. The Radio-Micronwter. 
By C. V. Boys, Assoc. Royal School of Mines, Demonstrator of Physics at the 
Science Schools, South Kensington. 
Communicated by Professor A. W. PtuCKER, F.R.S. 
Received Marcli 8,—Read April 19, 1888,—Revised January 5, 1889. 
In the preliminary note on the Badio-micrometer which I had the lionoiir to present 
to the Boyal Society last year (1887), I promised to complete, as far as I might be 
able, the development of the instrument, and, in case of any great improvement in the 
proportions of the parts, to exhibit an instrument in the improved form. In the 
present paper I have shown how the best sizes of the several parts may be determined, 
and how the best result may be attained. 
I must, however, first refer to the fact that on February 5, 1886, M. d’Arsonval 
showed, at a meeting of the Physical Society of France, an instrument called by him 
the Thermo-galvanometer, with which mine is in all essential respects identical. The 
invention of an instrument for measuring radiant heat, in which one junction of a 
closed thermo-electric circuit suspended in a strong magnetic field is exposed to 
radiation, is due entirely to M. d’Arsonval, and I need hardly say that it was in 
ignorance of the fact that he had preceded me that my communication was made 
to the Boyal Society. As soon as I became acquainted with M. d’Arsonval’s work, I 
took the earliest opportunity of admitting his claim to priority (see ‘ Nature,’ vol. 35, 
p. 549). 
I venture, however, to think that, although the difterences between M. d’Arsonval’s 
thermo-galvanometer and my radio-micrometer are essentially differences of detail, 
that even at the time of my original communication I had succeeded in producing the 
most sensitive instrument of practical utility, with the exception perhaps of the 
bolometer, which had up to that time been constructed for the measurement of radiant 
energy. As 1 hope to be able to show in the present communication that I have 
still further improved the proportions of the several parts, it may perhaps be fortunate 
that I was not aware that so able and ingenious a physicist had already made an 
instrument with the properties which I regarded as of so much importance, viz., the 
low resistance and small moment of inertia of the circuit, the small capacity for heat 
of the junction, the quickness and dead-beat character of the indications, and its 
freedom from extraneous influences. Had I known of M. d’Arsonval’kS work, I should 
probably have given no more attention to the idea. 
15.3.89 
