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Now, when a flash of lightning crosses the sky it suddenly expands the air all round it as it passes, so that globe after globe of sound-waves is formed at every point across which the lightning travels.
As is well known, it is electric currents and not sound-waves that are transmitted over a telephone circuit. The magneto-electric telephone in its simplest form consists of a pair of instruments called respectively the transmitter and the receiver. We talk into the transmitter and listen at the receiver.
The instrument must not only translate sound-waves into electric impulses, but change these back again into sound-waves; it must not only hear, but also speak! You remember our first fact in regard to sound: it is caused by motion. All that is needed to make anything speak is to cause it to move so as to give rise to just such air-waves as the voice makes. Mr.
Sound-waves for example light-rays, electricity these have been freely at our service from the beginning. Electricity might have been used ages ago, had not dull-witted man refused to find anything better for lighting purposes than an oil-lamp or a tallow candle! If, in past periods, he had been told 'there is something else' he would have laughed his informant to scorn.
In other words, with Bell's telephone the sound-waves themselves generate the electric impulses, which are hence extremely faint. With the Edison telephone, the sound-waves actuate an electric valve, so to speak, and permit variations in a current of any desired strength.
That shows you have some definite idea of it; but, after all, that answer is not the right one. The telephone does not convey sound. "What does its name mean, then?" do you ask? Simply, that it is a far-sounder; but that does not necessarily imply that it carries sounds afar. Strictly speaking, the telephone only changes sound-waves into waves of electricity and back again.
The varying pull on the plate must make it move, and this movement must set the air against the plate in motion in sound-waves corresponding exactly with the motion setting up the electric waves in the first place; in other words, the sound-motion in one telephone must be exactly reproduced as sound-waves in a similar instrument joined to it by wire.
In any case the sound-waves hitting against the wall will bound back from it just as a ball bounds back when thrown against anything, and so another set of sound-waves reflected from the wall will come back across the room.
When two telephones are connected by means of a wire, they act in this way, the first telephone changes the sound-waves it receives into electric impulses which travel along the wire until they reach the second telephone, here they are changed back to sound-waves exactly like those received by the first telephone.
Third the pitch, high or low, which is controlled by the distance from crest to crest of the sound-waves or, as we say, from node to node of the vibrations. To the most sensitive human ear, the highest limit of audibleness is reached by sound-waves estimated at twenty-eight-hundredths of an inch from node to node equal to 48,000 vibrations per second.
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