9G 



Mr. C. V. Boys. 



[Apr. 19, 



April 19, 1888. 



Admiral Sir Gr. H. RICHARDS, K.C.B., Vice-President, in the 



Chair. 



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

 for them. 



The Right Hon. Lord Sudeley was admitted into the Society. 

 The following Papers were read : — 



I. " The Radio-Micrometer." By C. V. Boys, A.R.S.M. Com- 

 municated by Professor A. W. RliCKER, F.R.S. Received 

 March 8, 1888. 



(Abstract.) 



In the full paper I have treated the subject of the radio-micrometer 

 in such a manner as to arrive at the best proportions of the instru- 

 ment. But I have first referred to the fact that the invention of an 

 instrument of the kind was originally made by M. d'Arsonval, and it 

 was in ignorance of this that I sent in my preliminary note. 



The instrument consists essentially of a thermo-electric circuit 

 suspended by a torsion fibre in a strong magnetic field. At first I 

 have shown that the parts cannot be too thin nor the circuit too small 

 until the limits imposed by practical considerations make further re- 

 duction objectionable. I have made the circuit of a bar of antimony 

 and bismuth, with the ends joined by a hoop of copper wire. 



I have at first taken the bar as an invariable, and shown how the 

 copper wire may be proportioned to it to give the best results. 



By " best " may be meant that which will give the greatest deflec- 

 tion, either for the weight or for the moment of inertia of the sus- 

 pended parts. 



Calling 



W the weight of the bar and mirror (invariable), 

 w the weight of the copper wire (variable), 

 C the resistance of the bar (invariable), 

 r the resistance of the copper wire (variable), 



I the length of the rectangle of copper, supposed 1 cm. wide, 

 u the weight of a piece of copper of unit dimensions, 



v the resistance of a piece of copper of unit dimensions, 

 a the sectional area of the copper wire, 



