50 Trot'. D. E. Hughes on an Induction-balance 



rapid vibratory diaphragm of the telephone and phonograph 

 of Professor Bell and Mr. Edison. For just as the receiving 

 diaphragm, from the non-preponderating character of its 

 very rapid natural vibrations, is able to produce truthfully 

 in miniature the varied sound-vibrations communicated to the 

 transmitting diaphragm, so does our seismograph truthfully 

 record in miniature the earthquake shocks it has received. 



And we are led to think that, if rapidity of vibration be com- 

 bined with viscous resistance (as explained in the paper) tele- 

 phones and phonographs may be successfully constructed of 

 far larger dimensions and of far greater power than has hi- 

 therto been attempted. 



V. Induction-balance and Experimental Researches therewith. 



By Prof. D. E. Hughes *. 



[Plate V.] 



IMMEDIATELY upon the announcement of Arago's dis- 

 covery of the influence of rotating plates of metal upon 

 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 circu- 

 lating in a metallic mass as to bring some new light to bear 

 on the molecular construction of metallic bodies. 



The question was particularly studied by Babbage, Sir John 

 Herschell, and 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 in- 

 duction-balance, to which he gave the name of " differential 

 inductors." In those days physicists did not possess the ex- 

 quisitely sensitive galvanometers and other means of research 

 that we possess today; 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 ex- 

 perimenting with the microphone I had ample occasion to 

 appreciate the exquisite sensitiveness of the telephone to 

 minute induced currents. This led me to study the question 

 of induction by aid of the telephone and microphone : the re- 

 sults of those researches have already been published. Con- 

 tinuing 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 compara- 

 * Communicated bv the Physical Society. 



