ELECTEO-MAGNETISM. 



435 



How many 

 wires are nec- 

 essary for 

 ■working the 

 telegraph ? 



round the magnet for a longer or shorter time a dot, or a line is made, and 

 the telegraphic alphabet consists of a series of such marks.* 



Grove's battery (see Fig. 3-40) is generally used for working the telegraph, 

 about thirty cups being required for a distance of 150 miles. These cups 

 may be kept in one compact space, but operate the telegraph more success- 

 fully when distributed along the line. Such batteries will work for about two 

 weeks without replenishing. 



Formerly two wires were required in telegraphing; one 



conveyed the current from the battery to the'electro-magne^ 



at a distance, through which it passed, and then returned by 



another wire back to the battery. At present but one wire is 



generally used. It was found that the earth itself might be 



made to perform the function of the returning wire. To effect this all that ia 



necessary is that one short wire from the battery at one end of a line, and 



from the electro-magnet at the other end, should be sunk into the moist 



earth, and there connected with a 

 mass of conducting metal, from 

 which the electricity passes to 

 complete the closed circuit 



For interrupting tlie 

 current and reoculatins: 

 the system of dots and 

 lines, an instrument call- 

 ed the Signal-key, or, 

 Break-piece, Fig. 3G8, 

 is employed. This is 

 placed near the battery, so as to be in the galvanic cir- 

 cuit. The operator, by pressing down the knob with tho 

 finger, closes the circuit and allows the current to pass, but 

 when the pressure is removed communication is interrupted 



* The following table exhibits the Eigns employed to represent letters In the Morso 

 ■ystem of telegraphing : 



Expprienced operators are often able to understand the message merely from the sounds, 

 or clicks, of the lever. 



