THE DOUBLE NEEDLE.] UNDULATORY FORCES. ELECTRO-MAGNETISM. 



201 



The connection of the wires is precisely similar to that 

 illustrated in the arrangement shown in a previous 

 page.* The ends of the wire proceeding from the coil, 

 may be immersed in two cups containing a little mer- 

 cury ; and the battery wires being held in the hand, 

 they can be dipped in either cup, so as to send the 

 current in any direction that may be desired. Now, by 

 tliis arrangement, the needles of each instrument can be 

 simultaneously deflected ; and if the code of signals 

 already given be pasted on either side of the coils of 

 each model, a complete single telegraph arrangement 

 will be produced. 



If one instrument be placed in any part of a house, 

 and the battery and the other model be placed in 

 another apartment, and two wires be extended between 

 them, telegraphic signals can be just as easily transmitted, 

 as is done, in the usual way, by the ordinary telegraphic 

 arrangements ; and our readers will learn more practical 

 knowledge of the subject in half-an-hour, than a perusal 

 of works on telegraphy can possibly afford them. 



Many persons are much surprised and puzzled at the 



extraordinary rapidity with which the signals are read 



by the persons in charge of the telegraphic instruments 



at a station. We may, however, easily illustrate the 



readiness thus exhibited, by reminding our readers, that 



any musical instrument is rapidly performed on, solely 



from continued practice of the musician. He has had at 



1 to learn the notes a slow and laborious process : 



itually, however, he learns to read and play music at 



t. In the same way, the telegraph clerk gradually 



ires a proficiency in reading the indications which 



tli deflections of a needle afford. Practice in this, as 



in all other matters, alone makes perfect ; and we may 



venture on a paraphrase of a well-known quotation, to 



illustrate our remarks "No one suddenly became an 



expert telegraphist. " 



It was found that the single needle apparatus, although 

 sufficient for stations between which messages were only 

 '^ionally sent, was too slow in its indications, as the 

 business of telegraphy developed itself ; and, eventually, 

 the double needle instrument was invented. In this, 

 two needles are employed ; and thus a word could be 

 spelt in half the time required by the single needle 

 arrangement. The principles of construction arc identical 

 in each instrument. The code of signals, however, has 

 undergone some modifications. The following engraving 

 illustrates the appearance of the double needle instru- 

 ment, as employed by the telegraphic companies on 



most of our railway lines omitting the code of signals 

 on the face of the instrument. 



In the above engraving, we observe the two indexes 

 which are connected with the needles inside the instru- 

 ment. Pegs are placed on each side of them, to restrain 

 their deflections ; and at the lower part are two handles 

 by the movement of which the current of electricity is 

 readily sent in any direction, f These handles have wires 

 attached to them, which come in contact with those of 

 the battery and the telegraph instrument ; and according 



See ante, p. 259; fig. 23. f Ante, p. 200 ' 



as the current is directed by them over the coils, so the 

 needles are deflected right and left. On some lines 

 of railway an alarum is used, which, by means of an 

 electro-magnet, sets a bell in action, and so calls the 

 attention of the telegraph clerk. A portion of the 

 current is diverted from the needle coils ; which, by 

 moving a detent on clockwork, rings the bell. This 

 continues until stopped by the attendant. 



Before proceeding to explain a variety of practical 

 details, we will illustrate the manner in which a message 

 sent from one station is indicated and deciphered ; and, 

 for this purpose, shall employ the code of signals adopted 

 in the single needle instrument already shown at a 

 previous page ; because, by so doing, we shall avoid 

 any confusion which might arise in employing the two- 

 needle code for our purpose. We shall presume that 

 each attendant is at his station, and in front of the 

 instrument ; that the bell lias been rung by the sender, 

 and heard by the receiver. For example's sake, we may 

 suppose the inquiry made " What weather have you ?" 

 The sender moves the handle of his instrument so as to 

 produce the proper deflection of the needle for each 

 letter of the message, as seen beneath. 



Now, to illustrate this more fully, we will give a 

 literal explanation of the W h a t 

 first word of the message 

 "What." Forthe"W," ^ \V \\ Jll 

 there are one deflection to w v, 



the right, and throe to the VV e a l 



For "h" two to the :/ \V \\ JH \V \V J 



left and two to the right. 



For "a," two to the left: H a v 



and, for" t," three to the \\j \\ J/ \\, 



right, and one to the left. 



Now, in the double needle You 



instrument, the combina- i i III I 



tions of both needles are " '> * 



ust'd at the same moment, and so much time is saved. 



It is customary to shorten the message as much as 

 possible by leaving out unnecessary words and letters ; 

 but these are practical details which scarcely concern the 

 general reader. Each station on the line has a definite 

 signal ; so that, although the bells may be ringing, and 

 the needles working at intermediate stations, the at- 

 tendant at the intended -station can at once perceive 

 that his attention alone is required. The clerks have 

 also an abbreviated code of signals for special and con 

 stantly recurring circumstances; and it is usual to 

 transmit a special signal from and to each station when 

 a word or sentence is completed, to signify that the 

 sami! is, or is not, understood. 



With respect to the rapidity of transmission of a 

 message, we may observe, that it depends on the quick- 

 ness of perception and intelligence of both receiver and 

 sender (except under circumstances over which neither 

 have any control, and of which we shall presently speak). 

 From 60 to 100 letters may easily be sent and read per 

 minute. 



Most persons are curious to know how long it takes to 

 send a message by means of the electric telegraph ; and 

 independent of the number of words, there are other 

 circumstances to which we must now draw attention. 

 As we have sufficiently explained the construction and 

 use of the needle instrument, we shall now refer to the 

 conducting wires ; and the observations we are about to 

 make, are equally applicable to every form of telegraph 

 which we shall hereafter describe. There are two 

 methods of connecting distant stations by means of wires. 

 The Electric Telegraph Company suspend theirs on poles 

 raised at the side of the railway. The wire, which is of 

 thick galvanised iron, rests, not on the wood of the poles, 

 but on glass or porcelain supports, which are contrived 

 to shoot off the water deposited by rain, dew, or fog, so as 

 to prevent a loss of power by deficient insulation of the 

 wires. The Magnetic Telegraph Company enclose their 

 wires first covered with gutta-percha, so as to keep 

 them electrically separate in iron tubes : these are do- 

 posited on the sides of the main road ; and at different 

 parts, the wires are raised above ground, in an iron post, so 



