222 Intelligence and Miscellaneous Articles, 



air near the cannon. Evidently a musical sound o£ low intensity 

 must be used for a correct determination of the velocity of sound. 

 — -Silliman's American Journal, Eebruarv 1879. 



RESEARCHES ON BELL'S TELEPHONE. BY HENRI DUFOUR. 



The principles on which the construction of Bell's telephone is 

 based are direct consequences of the phenomena of induction and 

 electromagnetism ; and from theoretical considerations all that 

 passes in that instrument can be foreseen. When it is employed 

 we are struck, on the one hand, with the minuteness of the vibra- 

 tory motions necessary to produce magnetic modifications of the 

 magnet and the induction resulting from it, and, on the other, with 

 the relatively great intensity of the sounds produced. 



It seemed to me that it would be interesting to verify upon a few 

 instruments the principal phenomena which theory enables us to 

 foresee, and to seek out some of the causes which may modify 

 them. 



The instruments employed were constructed by M. J. Cauderay, 

 at Lausanne. The mean length of the magnet was 127 millims., 

 the thickness of the vibrating plate from 0*159 to 0175 millim. 

 The induction-coil contained 46 metres of wire of 0*3 millim. 

 thickness. 



Intensity of the Currents. — The maximum intensity observable is 

 obtained by pressing on the vibrating plate so as to bring it into 

 contact with the soft-iron termination of the magnet ; the dis- 

 placement it thus undergoes is about 1 millim., and produced a 

 deflection of 7-8° upon the galvanometer which I used. An equal 

 deflection in the opposite direction is observed when the plate re- 

 sumes its initial position. 



The movement of the plate towards the magnet produced an in- 

 verse induced current in the three instruments which I tried, the 

 pole of the magnet being in fact behind the coil through which 

 passed the cylinder of soft iron. 



Eor the purpose of knowing if the two currents, direct and in- 

 verse, possess an appreciable difference of intensity, the wires of 

 the telephone were put into communication with two carbon elec- 

 trodes dipping in water, and which could be connected with the 

 galvanometer by means of a commutator. A great number of 

 vibrations of the plate were produced, so that a series of induced 

 currents, direct and inverse, traversed the liquid ; the electrodes 

 connected with the galvanometer gave no polarization-current. 

 From this we may conclude that the difference of intensity of the 

 two currents is very slight. In the construction of the telephone, 

 therefore, no account is to be taken of the action which this 

 difference may in time exert upon the magnetization of the bar. 



Two of the instruments employed had poles of opposite names 

 submitted to the action of the coil ; and when joined they worked 

 as well as those which are symmetric. 



Intensity of the Maynetism. — The variations of intensity of the 

 agnetism were ascertained in the following manner : — The north 



