On the Telephone. 75 
the to and fro motion of the iron disc. On speaking into the 
instrument vibrations of the disc are set up which give rise to 
magneto-electric currents in the coil of wire, one end of which is 
attached to the line wire. These electric pulsations produce 
corresponding changes in the intensity of the magnetic field at 
the receiving instrument, and hence give rise to motions of the 
iron disc analogous to those at the other end. 
The extraordinary feature of this instrument is its extreme 
sensitiveness. The amplitude of the vibrations of the transmitting 
disc must be wonderfully small, and yet every inflexion of the 
voice is faithfully transmitted. At the receiving end the ampli- 
tude of the vibrations of the iron disc is so small as to be 
immeasurable, nevertheless not a syllable is lost, and in the larger 
instruments the voice is heard at a distance of some feet from the 
instrument. 
The question arises, is the sound due to a molar or a molecular 
motion of the disc? Iron when magnetized and demagnetized 
gives rise to a peculiar click due to molecular changes, to which 
reference will be made in the sequel. No motion of the iron 
would be apparent in this case, but if the sound sprung from any 
motion of the disc as a whole, such as would arise from currents 
of sensible strength fluctuating through the coil beneath, then 
the motion of the disc might be capable of detection. Attaching 
a mirror to the disc and reflecting therefrom a ray of light, no 
motion of this reflected ray was visible on speaking through the 
instrument. Other arrangements were tried by the author of the 
present paper to test this question. A sensitive flame is an ex- 
tremely delicate acoustic re-agent. Removing the wooden mouth- 
piece of the telephone, a small metal chamber was substituted, into 
this chamber coal gas at high pressure was sent, and burnt from an 
orifice furnished with a suitable jet. The gas thus traversed the 
iron disc within two inches of which it issued as asensitive flame, 
so that any sensible motion of the disc would be detected by a dis- 
turbance of the flame. Although a flame of extreme sensitive- 
ness was obtained, no effect was produced on the flame by any 
sounds shouted or hissed into the distant and electrically-joined 
telephone. These results confirm an experiment of Professor Bell’s, 
who glued the iron disc to a thick block of wood, and in this way 
spoke through and heard by the telephone. Here any molar 
