204 



SCIENTIFIC NEW^S 



[Nov. 1st, li 



THE TELEPHONE: ITS PRINCIPLES, 

 CONSTRUCTION, & APPLICATION. 



IV. 



A MISTAKE occurs in the table of cycle of actions at the 

 end of the last article (page 182), which makes the 

 table somewhat obscure. The three headings in italics, which 

 mark the divisions between the electrical and the mechani- 

 cal portions of the cycle should read, ^^ Mechanical Operations" 

 "■Electrical Operations^'' "Mechanical Operations" — and the 

 word " operations " standing between " variations of cur- 

 rent," and " strength resulting in " should be omitted ; 

 the sentence then reading, " And consequent variations 

 of current strength resulting in variable magnetisation," 

 etc. 



Many good instruments have been invented, and are 

 more or less in use, which mostly differ in details only from 

 those which have been looked at. The Edison loud-speak- 

 ing receiver, which is based upon a quite different law to 

 any other, demands a few words. Edison discovered that 

 a metal wire or style, resting upon a damp, moving surface, 

 such as the chemical paper of the Bain telegraph instru- 

 ment, had a smaller dragging stress communicated to it 

 when a current passed from the style to the moist surface, 

 and that, if mounted on a pivot, with a spring acting upon 

 it against the drag of the paper, it will slip back every time 

 a current passes. The reason is that, when the current 

 passes, the liquid is decomposed, and a little film of gas is 

 formed between the metal and the paper, reducing the fric- 

 tion. As the amount of gas is proportional to the strength 

 of the current, the decrease in the friction varies with the 

 current. This action is an exceedingly sensitive one, and 

 Edison patented it for telegraphic use before the telephone 

 had made any practical appearance. 



As soon as the Bell telephone came out, Edison saw the 

 possibility of adapting his discovery, and patented a tele- 

 phone, in which the damp paper of the telegraph instrument 

 was replaced by a revolving cylinder of damp chalk, and the 

 style was attached to a telephone diaphragm. This instru- 

 ment is the loudest speaking telephone in use, but is rarely 

 employed, as the clockwork necessary for revolving the 

 cylinder, and the necessity for keeping it in a uniformly 

 moist condition, make it a rather troublesome piece of ap- 

 paratus to keep in order. For lecture demonstrations, such 

 as making a distant musical performance audible all over a 

 large room, it is very effective, and it owes its loud-speaking 

 power to the fact that the energy given to the diaphragm is 

 derived from the clockwork, and may be therefore much 

 greater than the electrical energy, which acts as a controlling 

 force only. The Edison receiver bears much the same re- 

 lation to the Bell receiver that a variable resistance trans- 

 mitter bears to the Bell receiver. 



The main principles underlying the action of telephones 

 and some practical forms of instruments having been 

 described in the preceding articles, some of the chief prac- 

 tical adaptations of the telephone remain for description. 



As these adaptations are mainly dependent upon details 

 of a technical nature, only a general description of the more 

 prominent arrangements can be given here. 



For the simplest use of the telephone as a means of com- 

 munication between two fixed points, it is obvious that it is 

 necessary to have at each end of the line a transmitter and 

 a receiver and some means of calling attention, as the tele- 

 phone itself is not sufficiently loud for that purpose. The 

 calling arrangement is usually an electric bell of some 

 description, and a key and battery to work the bell ; or it 

 may be a small magneto-electric machine in place of the 

 battery. The magneto-electric machine is really a small 

 dynamo, worked by a handle, and giving off an alternating 



current. The bell used with magneto machines is a special 

 form, having a magnetised armature, which is alternately 

 attracted and repelled by the electro-magnet, through which 

 the alternating currents passes, and causes a little hammer 

 mounted on it to strike two bells in rapid succession. The 

 magneto bell instrument is better than the battery bell, as 

 it reduces the number of batteries to be maintained, gives a 

 louder ring, and rings through far longer lines than any 

 moderate sized battery can operate. Some very lazy people, 

 however, object so the slight trouble of turning the handle, 

 and insist on having the battery form of instrument. 



Some form of switch has to be used to connect the line 

 either to the bell or the speaking apparatus. If microphone 

 transmitters are in use, the switch must also put the battery 

 in circuit with the primary wire of the induction coil when 

 in the speaking position, and cut it off when in the ringing 

 position. This action is necessary to prevent the battery 

 wearing out rapidly, as it would if constantly " short- 

 circuited," through the low resistance of the transmitter 

 and the primary wire of the induction coil. When the 

 apparatus is not in use, the switch is in the "ringing" 

 position, so that the bell is in circuit, and a call can be 

 received. 



The switch is usually worked by the weight of the 

 receiving instrument, which, when not in use, is hung on 

 a hook, and so pulls the switch into the " ringing " 

 position. When the telephone is lifted off the hook, a 

 spring pulls the switch into the " speaking " position. 

 In this way the operation of the instrument is made as 

 nearly automatic as possible, with a view, of course, to its 

 being readily used by unskilled persons. On a simple 

 telephone line, fitted in the manner described, and work- 







Fig. 13. Ringing Position — Telephone Receiver on 

 Hook. 



ing between two places, A and B, if A wants to speak to B, 

 he rings the latter's bell by means of the push or the magneto 

 handle. B, on hearing the call, responds by a similar 

 action, lifts his receiver off the hook, and applying it to his 

 ear, awaits A's communication. A, on receiving notice of 



