VOLTAIC ELEOrRICITY. 



281 



into little Blips of the brass on the chalk surface, and aa the 

 the brass strip is fastened to the centre of a flexible 



ii.ii.linijjra of mica, it is thrown into vibration and gives oat 



: ,'iwil sounds. A large number of telephones on this 

 ptin. -iplo were, a few years back, imported into England from 

 i i>y the Edison Telephone Company, but it was Boon 

 fiiiitiil Hint tho atmosphere acted upon the porous chalk, so as 

 to render the instrument at times almost useless. Tho effects 

 x'ivfii by an instrument in good order wore most surpri-iiitf, 

 the Bonvds being loud enough for several hundred persons to 

 hear at the same time. This form of receiver has long ago 

 given phiro to the original Bell telephone. 



Tho telephonic system is far more used in America than it is 

 in Great Britain ; but in London and other large centres of 

 industry it is extensively employed. In London alone tho 

 T'-li'jihone Company numbers some thousands of subscribers, 

 who each pay so much for tho privilege of being in communica- 

 tion with tho Telephone Exchange. The wires from tho 

 different subscribers meet at a central office, where, by simple 

 mechanical arrangements, any two can be pat in direct com- 

 munication. Each subscriber has a number, and a directory 

 giving the names and numbers of all tho others. Supposing, 

 now, that Brown, at one end of London, wishes to ask a 

 question of his solicitor, Smith, at the other end of the city. 

 He first of all looks out for Smith in his directory, under tho 

 heading " Solicitors," and notes his number. Ho next touches 

 his telephone bell, and immediately from the exchange comes 

 an answering inquiry as to what he wants. " Put mo into 

 communication with number so and so," he replies, quoting 

 Smith's number. This is at once done, and lawyer and 

 client have a little conversation together, quite unheard by the 

 clerk at tho exchange, or any one else. In a busy city office tho 

 saving effected by this wonderful instrument must bo immense 

 while the total cost of tho convenience is calculated to be 

 something under one penny per message. 



We have now to notice another curious contrivance, which, 

 although of secondary importance from a practical point of 

 view, is full of interest, and has already found useful appli- 

 cations. The microphone, which represents a discovery rather 

 than an invention, was the result of some experiments con- 

 ducted only a few years ago by Professor Hughes. It will be 

 presently seen that this discovery depended upon the telephone 

 for its realisation, for the phenomena exhibited can only be 

 revealed through the medium of that instrument. If the one 

 has benefited the other in this way, we must admit that the 

 debt has been somewhat repaid, for until tho production of 

 the microphone no one was aware what a detector of minute 

 variations in the strength of tiny currents of electricity tho 

 telephone really is. 



It will be remembered that Bell's telephone receiver is quite 

 independent of a battery. Its contained magnet arouses feeble 

 currents of electricity in the coil of wire surrounding it, and 

 these currents are sufficient to actuate the movements of the 

 iron diaphragm which reproduces the original sounds. Pro- 

 fessor Hughes, in experimenting with the instrument, was led 

 to try the effect of including a battery cell in circuit with it. 

 Keeping the telephone to his ear, he noticed that directly the 

 circuit was made complete, the instrument recorded tho fact by 

 a loud click. Tho sumo thing happened whenever the circuit 

 was broken by detaching the battery from it. Continuing his 

 experiments, he submitted a thin wire, forming part of the 

 circuit, to a gradual strain by adding weights to it until tho 

 breaking-point was reached. Ho noticed that just before this 



risis occurred the telephone gave a warning sound, a kind of 

 grating noise, as if the particles of metal composing the wire 

 were dragged over one another previous to rupture. Subse- 

 quently ho discovered that the broken ends of such a circuit, 

 loosely connected together, would actually take up all kinds 

 of sonorous vibrations, which were duly reproduced in the 

 telephone. 



It now became obvious to Professor Hughes that he had hit 

 upon a most extraordinary detector of minute sounds, and he 

 at once endeavoured to find the best form for it to take. It 

 was clear that any loose contact between two conductors would 

 serve to transmit sounds, if a telephone and battery were 

 included in tho circuit. A little heap of metal chain, a pile of 

 nails or shot, different metallic powders, willow charcoal, the 

 pores of which had been filled up with mercury, were tried, 



and gave varying degrees of success. But thaw were all 

 gradually forsaken in favour of that form of carbon which u 

 commonly used for electric lighting purpose*. 



There are several forms of microphone which produce 

 extraordinary effects. Not only will the little instrument take 

 up vibrations which are quite 

 inaudible by any other means, 

 but it will most strangely 

 magnify them. Indeed, the mi- 

 crophone is to the ear what 

 tho microscope is to the eye. 

 Further than this, it will act as 

 a telephonic transmitter, and 

 will convey the complex articu- 

 lations of the human voice with 

 surprising exactness. Not the 

 least wonderful part of the in- 

 strument is its extreme simpli- 

 city. When Professor Hughes 

 described his dis- 

 covery to the 

 several learned 

 societies he pro- 

 duced a battery 

 made of three 

 little pickle-bot- 

 tles, a few match- 

 boxes, with car- 



Fig. 91. 



bon blocks fixed upon them with sealing-wax. A few yards 

 of copper-wire, a home-made telephone, and a few other simple 

 accessories, the entire cost of which would be dear at one 

 shilling, completed his stock-in-trade. The extremely homely 

 nature of the apparatus with which so much genuine work was 

 accomplished might afford a useful lesson to those who think 

 that nothing great can bo done without the French polish and 

 lacquered brass of the optician's shop. 



There are two forms of microphone, both of the most simple 

 construction, but which give astonishing results. In one, 

 which is illustrated at Fig. 91, a carbon pencil about an inch 

 in length is pointed at each end like a cigar. These points 

 fit loosely into hollows prepared for them in two little square 

 blocks of carbon, which are fastened by cement to a vertical 

 piece of board. A base board of the same material, or, better 

 still, a box with an open end, supporting this upright, upon 

 which the blocks are fixed, completes the instrument. The 

 telephone and battery are placed in circuit with it by attaching 

 wires to each of the carbon blocks. 



In the other form of instrument the carbon rod is placed in 

 a horizontal position (see Fig. 92). This rod has a hole drilled 

 through its centre to make room for a short length of brass 

 wire, which forms a balance-pin upon which it rests. One end 



Fig. 92. 



of the rod should slightly overbalance the other, so that the 

 heavier end can just lightly rest upon a little block of carbon. 

 Tho piece of wire upon which it is pivoted is hung in the centre 

 of a U-shaped piece of brass. One of the current wirea is 

 attached to this support, and the other to the fixed carbon 

 block. This last arrangement of the instrument will be found 

 especially sensitive, and the base board can conveniently be 

 prolonged, so that different things, which may from time to 

 time form the subject of experiment, can be laid upon it. 

 Thus a watch may be placed there, while the wires can be 



