56 



Prof. D. E. Hughes. 



[May 15, 



III. " On an Induction-Currents Balance, and Experimental Re- 

 searches made therewith." By Professor D. E. Hughes. 

 Communicated by Professor Huxley, Sec. R.S. Received 

 May 5, 1879. 



Immediately upon the announcement of Arago's discovery of the 

 influence of rotating plates of metal npon a magnetic needle (1824), 

 and Faraday's important discovery of voltaic and magneto-induction 

 (1831), it became evident that the induced currents, circulating in a 

 metallic mass, might be so acted upon either by voltaic or induced 

 currents as to bring some new light to bear on the molecular construc- 

 tion of metallic bodies.. 



The question was particularly studied by Babbage, Sir John Herschel, 

 and by M. Dove* who constructed an induction balance, wherein 

 two separate induction coils, each having its primary and secondary 

 coils, were joined together in such a manner that the induced current 

 in one coil was made to neutralize the induced current in the opposite 

 coil, thus forming an induction balance, to which he gave the name 

 of differential inductor. In those days physicists did not possess the 

 exquisitely sensitive galvanometers and other means of research that 

 we possess at the present day, but sufficiently important results were 

 obtained to prove that a vast field of research would be opened if a 

 perfect induction balance could be found, together with a means of 

 correctly estimating the results obtained. In experimenting with the 

 microphone I had ample occasion to appreciate the exquisite sensitive- 

 ness of the telephone to minute induced currents. This led me to 

 study the question of induction by aid of the telephone and micro- 

 phone. The results of those researches have been already published. f 



Continuing this line of inquiry, I thought I might again attempt 

 to investigate the molecular construction of metals and alloys, and 

 with this object I have obtained, after numerous comparative failures, 

 a perfect induction balance which is not only exquisitely sensitive 

 and exact, but allows us to obtain direct comparative measures of the 

 force or disturbances produced by the introduction of any metal or 

 conductor. 



The instrument which I have the honour to present to the Royal 

 Society this evening, consists, 1st, of the new induction- currents 

 balance ; 2nd, microphone, with a clock as a source of sound ; 3rd, 

 electric sonometer, or absolute sound measurer, a late invention of my 

 own ; 4th, a receiving telephone and three elements of Daniells' 

 battery. 



* Pe la Eire, " Treatise on Electricity," vol. i, chap. v. London, 1853. 

 f " Comptes Eendus," December 30, 1878, and January 20, 1879 ; Society of 

 Telegraph Engineers, March 12, 1879. 



