242 



ELECTRO-TELEGRAPHY 



Tho figures on the right show the number of marks or signs usod in printing tho snino 

 word in each kind of telegraph. 



Herring's Telegraph Alphabet, 



EISHAUVWJ RPLF 



........... I -I -I -II -III -I- -II- -I- ! 



TMO 



I II II 



NDB 



GZK XY Q 



H- I- I- I- II- II- l-l M l-ll ll-l 



The Rheo-clectro-static system of telegraphy was first described by M. Botto, in 1848. 

 It is applicable to some, but not to all forms of telegraph. It has been applied on the 

 South-Eastern Kail way to the signal bells (fig. 814), for the purpose of reducing the 

 amount, of battery power required under other circumstances to be maintained. The 

 wire, by which a pair of bells are connected, is in its normal state in permanent con- 

 nection with the similar polo, say tho positive, of batteries of equal power at the re- 

 spective stations, so that two currents of equal power are opposed to and balanced 

 against each other. Under these circumstances, the wire is in a null, or rheo-clcctro- 

 static state ; neither current circulates. If tho connection of one of the batteries is 

 reversed, so that its negative pole is presented to tho wire, then tho currents of both 

 batteries are in the same direction, and they circulate as one current, equal in value 

 to tho combined force of tho two batteries. The application is obvious ; that, whereas, 

 under the ordinary system, a whole battery, of force sufficient to traverse the distance 

 and do effective work, must be at each station, under this system only half such buttery 

 is necessary at each station for producing tho same effective work. Also, if a Little 

 more battery power is placed at each station than is necessary for tho actual work 

 required, signals of higher power are obtained under common circumstann s ; and 

 also tho equilibrium of the two opposed currents may be disturbed at any place 

 between the two stations, and signals m;iy IK- madtj by men 1 y making a connection 

 between tho line-wire and the earth ; because the negative pole at each station is lilted 

 up in permanent connection with tho earth; and, as tho positive poles arc in like 

 connection with the line-wire, each battery current is made to circulate through its 

 own signal-bell every time tho earth and lino-wire are placed in connection. Uy this 

 means the guard of a train can make signals of distress to the nearest station without 

 the aid of portable apparatus. Considerable caro is required to obtain good communi- 

 cation with tho earth on the open railway for making distress signals, or otherwise tho 

 discharge is imperfect, and no signal is made. Fish-jointed rails are very valuaMe fur 

 this purpose ; in their abst nee, (specially at embankments, metal must bo buried tor 

 the purpose at intervals in tho moist earth, and a wiro attached for use. Contact 

 springs on the telegraph poles are pr,,; 



Telegraph wires are suspended to poles by insulators of earthen ware, p 1 

 porcelain ; the material and shape varying according to tho experience of the cnirineer 

 and tho length of line to bo insulated. In very short lengths, tho battery power re- 

 quired for overcoming the resistance is not ^reat : it wi 11 therefore not, overcome tho 

 resistance of an insulator of moderate quality, and escape to the polo and thence to 



