1877.] B. S. Brough — Pro/ Oraham BelVs TelepJione. 255 



position, in virtue of its elasticity, and its potential energy is converted 

 into the kinetic form of an electric current transmitted in the reverse 

 direction to the first. 



The magnitude of the electromotive force produced in the coil will be 

 proportional to the number of lines of force cut through per unit of time ; 

 and will, therefore, be clearly proportional to the rate of displacement of 

 the diaphragm B, and thence to the energy of the impact we impress on 

 the disc B. 



Small impacts will produce small electromotive forces : large impacts 

 large electromotive forces. 



Moreover, the inertia of the diaphragm is so small, that it is always 

 ready to receive fresh impressions, which will be simply super-imposed on 

 •those already existing. 



Lastly, the iron core A being so short relatively to its diameter, and 

 being initially so highly magnetized, readily receives and loses the small 

 accessions of magnetism to which it is subjected. 



The result of all this is, that variations of jDressure on the disc B will 

 alwaj^s give rise to electromotive forces proportional to their magnitude. 



Thus if we soxmd a note in front of the disc J5, not only will it impart 

 its fundamental vibration to the disc, but also the subsidiary vibrations re- 

 presenting its harmonics. 



Hence, not only will a principal periodic electromotive force, corre- 

 sponding to the fundamental vibration of the note, be generated in the coil 

 C, but also minor electromotive forces, corresponding to the harmonics. 



And, finally, not only will a series of principal current waves, corre- 

 sponding to the fundamental vibration, be sent to the line, but on their con- 

 tour will be impressed the minor undulations of electrical potential repre- 

 senting the timbre of the original note. 



The action of the apparatus in the receiving station will be at once 

 apparent. There, the received currents flowing through the coil of wire, 

 in their turn re-act on the diaphragm B. The diaphragm, like the mirror 

 of Thomson's Cable Galvanometer, has no fixed zero, but is ready at every 

 moment to follow each wave. The motion of the diaphragm sets the air 

 in vibration, and reproduces the original note. 



The- sensibility of the apparatus as a receiving instrument is extraordi- 

 nary. Certainly the strongest current with which it is at any moment 



worked does not exceed . of the centimetre-gramme-second 



A- y\J\J\J *\J\J\J *\J\J\J 



unit current. The current with which our relays are worked in India is 

 400,000 times as strong. 



The practical objection to the instrument in its present form is that 

 the forces concerned are so microscopic. The consequence is that the 



