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THE POPULAR EDUCATOR. 



however imperfect when viewed from a modern standpoint, must 

 ever be regarded as the pioneer of telephony. Eeiss's telephone 

 consisted of a box, with a mouthpiece at one side, through 

 which a person might sing, and with a hole at the top filled in 

 with a parchment membrane which vibrated in sympathy with 

 the singer's v.oice. To the centre of this parchment was 

 attached a small metallic wafer, in connection with one pole of 

 a battery. Exactly over this wafer, and nearly touching it, 

 was a wire leading to one of Page's electro-magnetic bars, and 

 thence to the other pole of the battery. We may suppose 

 that the bar was in an apartment far away from the parch- 

 ment-covered box ; indeed, we may look upon one as the 

 transmitter of sounds, and the other as receiver. The operator 

 singing a note into the box say a note giving 500 vibrations 

 per second would cause the parchment to vibrate at the same 

 rate. So the wafer would be brought against the wire above it 

 500 times per second, and consequently the electrical contacts 

 would be the same in number. The bar would, therefore, give 

 500 metallic clicks per second, which would resolve themselves 

 into the same note sung to the transmitter. In this way any 

 note or song could be accurately reproduced at a distance from 

 the singer. Such a telephone may now be regarded as a 

 very interesting curiosity; but let it be remembered that it 



electricity as the most probable means of accomplishing such a 

 result, for by it distance and time seem to be annihilated. 



To Professor Graham Bell is due the invention of the first 

 articulating telephone ; and it speaks much for the perfection of 

 the instrument when we state that it is now in common use all 

 over the world, in the same form as it originally left its dis- 

 coverer's hands. Like the little " thread telephone " above 

 described, it will act both as transmitter and receiver ; but, 

 unlike it, its carrying power is not limited to a few yards, 

 for it has maintained communication between two places one 

 thousand miles apart. 



In actual practice Bell's telephone is not used as a transmitter, 

 although it is almost exclusively used as a receiver, for the sounds 

 given are very weak. But by employing an independent trans- 

 mitter, of which there are many different forms, and a battery, 

 the sounds given are much amplified. A battery, too, is essen- 

 tial for giving a warning signal by means of an ordinary electric 

 bell. However, to make our description of Bell's telephone 

 clear, we shall treat it as if it were worked between two 

 stations, both as transmitter and receiver, and shall leave out 

 all consideration of the usual supplementary apparatus. 



Bell's telephone consists essentially of three parts a magnet, 

 a coil, and a vibrating diaphragm of metal. These are encfosed 



Fig. 89. SECTION OF BELL'S TELEPHONE. 



would only reproduce a musical note ; it would not transmit 

 articulate speech. It may be described as a tone telephone, to 

 distinguish it from an articulating telephone. 



By a very simple contrivance we can easily prove that a 

 vibrating plate of metal, parchment, or even paper, can repro- 

 duce speech in a most perfect manner. We merely want two 

 round boxes with thin metal bottoms, or, better still, boxes with 

 bottoms formed of parchment, or parchment-paper, and, say, 

 50 feet of fine twine. The ends of this twine are securely 

 fastened to the parchment bottoms of the boxes by boring a 

 hole in the centre of each and knotting the string. Now let 

 two persons stand each with a box in his hand, and as far 

 distant from each other as the string will allow, taking care 

 that the line is kept taut. If one person now speaks into his 

 box, the sounds are exactly reproduced in the other one. 



Now let us briefly consider the theory of this simple form of 

 telephone, which has been called the " thread-telephone," also 

 "the lovers' telegraph." The sound-waves created by- the 

 speaker's voice throw the parchment of the transmitter into 

 active vibration ; in other words, the parchment executes a 

 rapid to-and-fro motion. The tight string leading to the other 

 parchment diaphragm on the distant box causes that also to be 

 pulled into exactly the same movements, so that the same 

 impulses are given back to the air, and the sounds are repro- 

 duced. It is quite evident that this form of instrument must 

 be of limited application, for the communicating cord or wire 

 must be kept straight and tight, and must be made of some 

 material not liable to change from heat, cold, moisture, etc. 

 But it affords proof of the phenomenon that a vibrating plate 

 will take up and give back all kinds of sounds, and even the 

 wonderful complexities involved in articulate speech. It follows 

 that if we can by any means cause the movements of one plate 

 to be repeated at any distance by another one, we shall at once 

 have an improvement on the ordinary telegraph. We look to 



in a case about eight inches long somewhat of the form of a 

 skipping-rope handle which can be conveniently held in the 

 hand and placed to the mouth or ear according as it is used as 

 transmitter or receiver. The magnet is of the bar form, about 

 six inches long by three-eighths of an inch in diameter. One 

 end is surrounded by a coil of silk-covered copper-wire the 

 ends of which are carried to the end of the case for ready con- 

 nection with the line wires. Facing the coil-crowned end of 

 the magnet, and all but touching it, is a diaphragm of thin iron 

 enamelled plate, such as is used by itinerant photograpers for 

 the production of those portraits known as "ferro-types."' The 

 forward end of the case, where this diaphragm is placed, is 

 trumpet-mouthed, and through the small orifice in the centre is 

 seen its surface. Let us suppose now thafc two such instruments 

 are at a distance of about one mile apart, joined up with wires 

 ready for operation. A person speaking into one will be readily 

 heard by a person listening at the other, and the explanation 

 of the working of the instrument can be thus explained. 



The speaker causes the diaphragm of his instrument to 

 vibrate. Immediately underneath this diaphragm is a magnet, 

 surrounded by a coil of wire. Now we have learnt in a previous 

 lesson that a magnet so situated will induce currents of 

 electricity in such a coil, if it be moved backwards and forwards. 

 We can, therefore, readily understand how a piece of iron 

 moving in close proximity to such a combination will cause 

 rapid alternations in the strength of the currents flowing 

 through the system, which will affect the distant receiver. The 

 distant magnet will also be subject to these variations of 

 strength, with the result that its iron diaphragm will be thrown 

 into the same vibrations as that of the transmitting agent. 

 Therefore, as in the thread telephone, the original sounds are 

 reproduced. Fig. 89 shows Professor Graham Bell's telephone in 

 elevation and section, and the position of the various parts will 

 be readily understood from what has gone before. 



