TELEGRAPH WORKING. 



237 



fourth eight, or thirty-two signals, which are found to be sufficient for every 

 purpose, and by practice may be both produced and read off with facility. 

 Before a message is about to be delivered the commutator is so placed as 

 to ring a bell, which is done by the same arrangement as in a common 

 alarm-clock, but the action is set in motion by a peculiar contrivance, which 

 depends upon the property a bar of soft iron has of becoming magnetic 

 when a wire is wound round it and a current of electricity passed through 

 this wire; this magnetic property exists only as long as the current passes, 

 and stops the instant it is cut off. The catch of the alarm is disengaged by 



Fig. 249. Handles and needles of telegraph. 



the movement of a bar of iron being drawn to the magnet while the current 

 passes, and forced back again by a spring when it is stopped, thus setting in 

 action the mechanism of the alarm ; or in some cases there is a simple 

 contrivance for causing a rapid flow and stoppage of the electricity, so that 

 the bar is alternately attracted by the magnet and released by the spring, 

 and this motion rings the bell as long as it is continued. The bell is always 

 rung to give notice that a message is about to be sent, and at the station 

 where it rings, the bell at the former station is rung in return, to show that 

 they are prepared to receive the message : which is then spelt, letter after 



Fig. 250. Telegraph wires. Fig. 251. Insulator. 



letter, by moving the handles into the proper positions, and as it is being 

 sent, the eye is kept on the dials, certain single signs are made and recognised, 

 which will communicate any reply from the station to which the message is 

 being sent, such as " repeat," or " not understood." The wires which 

 convey the electricity are made of galvanised iron (iron coated with zinc), 

 and as they must be kept from all communication with the earth by some 

 substance incapable of conducting it, they are therefore stretched between 

 wooden poles (fig. 250), and rest upon sockets or supports of glass or glazed 

 earthenware, which are both substances incapable of conducting the electricity 



