V. The radio-micrometer

In the preliminary note on the Radio-micrometer which I had the honour to present to the Royal 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 Royal 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).