THE TELEPHONE. 747 



Now, the telephone of Prof. Graham Bell is an articulating telephone, 

 which can transmit not only melodies sung to it, but ordinary speech, and 

 that so faithfully that we can often recognise the speaker by his voice as 

 heard through the telephone. How is this effected ? It is manifest that if by 

 any means we can cause the tinned plate of the receiving instrument to vibrate 

 in precisely the same manner as that of the transmitter, the impression on the 

 ear will be exactly the same as if it had been placed at the back of the 

 plate of the transmitter, and the words will be heard as if spoken at the 

 other side of a tinned plate. 



But this implies an exact correspondence, not only in the number of 

 vibrations, but in the type of each vibration. 



Now, if the electrical part of the process consisted merely of alternations 

 between current and no current, the receiving instrument could never elicit 

 from it the semblance of articulate speech. If the alternations were sufficiently 

 regular, they would produce a sound of a recognisable pitch, which would be 

 very rough music if the pitch were low, but might be less unendurable if the 

 pitch were high ; still, at the best, it would be like playing a violin with a 

 saw instead of a bow. 



What we want is not a sudden starting and stopping of the current, but 

 a continuous rise and fall of the current, corresponding in every gradation and 

 inflexion to the motion of the air agitated by the voice of the speaker. 



Prof. Graham Bell has recounted the many unsuccessful attempts which he 

 made to produce undulatory currents instead of mere intermittent ones. He 

 had, of course, to give up altogether the method of making and breaking 

 contact. Every method involving impact of any kind, whether between electric 

 contact pieces or between the sounding parts of the instrument, introduces 

 discontinuity of motion, and therefore precludes a faithful reproduction of 

 speech. 



In the ultimate form which the telephone in his hands assumed, the electric 

 current is not merely regulated but actually generated by the aerial vibrations 

 themselves. 



The electric principle involved in Bell's telephone is that of the induction 

 of electric currents discovered by Faraday in 1831. Faraday's own statement 

 of this principle has been before the scientific world for nearly half a century, 

 but has never been improved upon. 



Consider first a conducting circuit, that is to say, a wire which after any 



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