GALVANISM. 



dot will be imprinted, and if for a longer time, 

 a line or dash. By a combination of dots and 

 dashes, an alphabet is formed, the letters oftenest 

 used being most easily signalled. Thus, E is one 

 dot, and T is one dash ; A is a dot and a dash, 

 and I is two dots ; and so on. But clerks do not 

 spell out every word ; they have a great many 

 contractions, universally recognised. The paper 

 is supplied from a large roll or bobbin (not seen in 

 the figure) above the instrument, which turns round 

 as the rollers demand. Of course, it is rolled off 

 only while a message is being sent. When the 

 clerk receives the signal for a message, he pulls 

 out the switch, S, which liberates the clockwork, 

 and sets the paper in motion. An experienced tran- 

 scriber could easily make out the message though 

 the paper never moved. In practice, he requires 

 to look at the paper only when he has heard indis- 

 tinctly, for the mere clicking of the lever has become 

 to him a language perfectly intelligible, and it is 

 much easier to hear and write than to look and 

 write. But the printing of the message as well, 

 relieves him of all responsibility as to its accuracy. 

 The transmitting key or communicator is seen 

 in fig. 35. It is a brass lever, //, with two nipples, 

 ;, n, one or other of which is in contact with one 

 or other of the projections, a, b, according as H 

 is pressed or not. The lever is in permanent 

 connection with the line-wire L, a with the copper 

 pole of the local battery, and b with the local 



\ T ' 



Fig. 35. 



' Morse.' It is obvious that when the lever is left 

 to itself (as in the fig.), it is in the receiving posi- 

 tion, for a current from the distant station can 

 reach the local register. But if m be pressed 

 down on a, it will be in the sending position, for 

 the local battery is put in connection with the 

 line-wire. Of course, if both keys were pressed at 

 the same time at two stations, no message could 

 pass between them. The lever has a play of not 

 over one-tenth of an inch on each side. This is 

 quite sufficient to break contact, and it allows a 

 very rapid manipulation. An expert telegrapher 

 can transmit from thirty to forty words a minute. 

 How two Stations are connected. The manner 



system will be seen from fig. 36. Each station, 

 S, S,, is provided with its own communicator, in- 

 dicator, and battery, included in the circuit in such 

 a way that only one communicator and its battery, 

 and the opposite Morse, can be in circuit at the 

 same time. In the figure, S is supposed to be 

 sending a message to S,. Its key, k, is depressed, 

 so that the current flows from the positive pole, C, 

 of its battery to the key, and thence to a galvan- 

 ometer , included in the circuit, to shew the clerk 

 that a current really passes. From g it passes 

 through the wire LL to the receiving station S,, 

 where it passes first through a galvanometer, and 

 thence to the key. The key, being in its natural 

 position, allows the current to pass to the Morse, 

 and there record its variations. Thence it passes 

 to the earth-plate P a , and is lost, while an equal 

 amount of negative electricity is lost from the 

 zinc pole at P. 



If k were left to itself, and tf depressed, then S, 

 would be the sending, and S the receiving station, 

 and the connections would be exactly as in the 

 figure, only at opposite stations. When the clerk 

 at S wishes to telegraph to Si, he depresses the 

 key several times, and sends a series of dots and 

 dashes, giving the name of the station. The 

 attention of Sj is arrested by the clicking of 

 the armature of his Morse, or, in some cases, by 

 an alarm, which is made to ring till S x come and 

 throw it out of circuit. He thereupon sends back 

 to S that he is ready, and sets the clockwork in 

 motion for the printing to begin. 



Relay. Hitherto, we have supposed that the 

 recorder or Morse is worked directly by the line- 

 current. This is only done on short circuits of 

 less than fifty miles. There is so much leakage 

 on a long line, that but a very feeble current 



Fig. 36. 



Fig. 37- 



reaches the distant station ; and to employ battery- 

 power sufficient for direct working of the Morse, 

 would be very expensive. In place, therefore, of 

 passing the line - current through the 

 Morse, it is passed through a separate 

 instrument, called a ' relay,' which is much 

 more sensitive than the Morse. It is 

 shewn in fig. 37 ; and is simply an electro- 

 magnet, E, with coils of very long fine wire, 

 and an oscillating armature - lever, *A/. 

 A very feeble current is sufficient to mag- 

 netise the core of E, and to attract A, so 

 that the interruptions of the current by the 

 sending clerk will produce simply a series 

 of oscillations of eM. Now, the zinc pole 

 of the local battery is connected with the 

 metal pillar N, and also with the screw S. 

 but not with the screw S', which is insulated 



which two stations are 'joined up' on Morse's I from the pillar. A wire from the copper pole of the 



