THE TELEPHONE. 745 



is transmitted. The copper remains at rest, but a variable electric current 

 flows to and fro in the circuit. 



It is this which distinguishes the electric telephone from the ordinary 

 speaking tube, and from the transmission of vibrations along wooden rods by 

 which Sir Charles Wheatstone used to cause musical instruments to sound in 

 a mysterious manner without any visible performer. 



On the other hand, we have to distinguish the principle of the articulating 

 telephone from that of a great number of electrical contrivances which produce 

 visible or audible signals at a distance. Most of these depend on the alternate 

 transmission and interruption of an electric current. In some part of the 

 circuit a piece of apparatus is introduced corresponding to this instrument which 

 is called a key. Whenever two pieces of metal, called the contact pieces, touch 

 each other, the current flows from the one to the other, and so round the 

 circuit. Whenever the contact pieces are separated the current is interrupted, 

 and the effects of this alternation of current and no current may be made to 

 produce signals at any other part of the circuit. 



In the Morse system of signalling, currents of longer and of shorter duration 

 are called dashes and dots respectively, and by combinations of these the 

 symbols of letters are formed. The rate at which these little currents succeed 

 one another depends on the rate at which the operator can work the key, 

 and may be increased by mechanical methods till the receiving clerk can no 

 longer distinguish the symbols. 



But the capability of the telegraph wire for transmitting signals is by no 

 means exhausted ; as the rapidity of the succession is increased, the ear ceases 

 to distinguish them as separate signals, but begins to recognise the impression 

 it receives as that of a musical tone, the pitch of which depends on the 

 number of currents in a second. 



Tuning forks driven by electricity were used by Helmholtz in his researches 

 on the vowel sounds, and the periodically intermittent current which they 

 furnish is recognised as a most valuable agent in physical and physiological 

 research. The tuning forks are of the most massive construction, and the 

 succession of currents goes on with the most inflexible regularity, so that 

 whenever we have occasion to follow the march of a process which takes place 

 in a short time, such as the vibration of a violin string or the twitch of a 

 living muscle, the tuning fork becomes our appropriate timepiece. 



Apparatus of this kind, however, the merit of which is its regularity, is quite 



VOL. II. 94 



