of Edinburgh , Session 1877-78. 
555 
4. On the Theory of the Telephone. By 
Prof. George Forbes. 
The Telephone, invented by Mr Graham Bell, is an instrument 
by means of which any sounds, musical notes or spoken sentences, 
sounded into an instrument at the sending end of a telegraphic wire 
may he reproduced at the receiving end. The theory of the Telephone 
is two-fold. First, the mechanical theory of the nature of sounds 
and of speech ; and secondly, the theory of the action of the instru- 
ment. The first part is well known. All sounds consist of a 
succession of waves propagated through the air, the rate and 
intensity of their succession determining the nature of the sound, in 
pitch, loudness, and tone. If the same succession of waves as a 
speaker makes in using his voice can be reproduced in the air in 
contact with the ear of any other person, by any means whatever, 
the latter person will hear a fac-simile of the sound uttered by the 
former. A theory of the action of the Telephone is distinctly given 
by the inventor in the specification of his patent. The receiving 
and sending instruments are identical. Pound the end of a bar 
magnet a coil of fine wire is wound, connected with a telegraph 
wire on the one hand and the earth on the other. In front of these 
a thin iron plate is caused to vibrate by the sounds uttered in its 
neighbourhood. Consequently, it approaches and recedes from the 
pole of the magnet. This alters the magnetism in the neighbour- 
hood. In Faraday’s language, 4 the lines of force are altered in 
position, so as to go across the coil of wire. ’ In consequence of this, 
by the well-known laws of electromagnetic action, a current of 
electricity is sent in one direction with each approach of the vibrating 
plate, and in the opposite direction with each recession. This suc- 
cession of currents reaches the receiving end with a strength pro- 
portioned to the extent of vibration of the iron plate. Here they 
circulate through the coil, and so intensify or diminish the magnetism 
of the bar. Thus, the vibrating plate at the receiving end is more 
or less attracted, according to the direction of the current. Hence, 
the plate at the receiving end vibrates to and fro in vibrations which 
are synchro nus and proportionate to those at the sending end ; and 
so the sounds are reproduced. 
