﻿THE MAGNETIC TELEGRAPH. 



extended to any distance, and the effect will be the same. I have no 

 doubt, that, if it were possible to carry them completely round the 

 world, and a current sufficiently strong were created, the result would 

 be similar. Wonderful as it may appear to you, the time required for 

 the electricity to pass from the copper plate to the zinc one, whether 

 the wires are long or short, is, apparently, the same ; nor can any 

 time-piece, however correct, measure so small a space ! When the 

 wires are very long, however, it is necessary to increase the power 

 or intensity of the current. This is done by uniting from ten to 

 fifteen or twenty pairs of plates, by connecting the copper plate of 

 each pair, or vessel, with the zinc plate in the next. In working the 

 telegraph, Groves' battery is used. It is made differently from the 

 one I have explained to you, but the principle is the same. The 

 wires from the battery are extended along, from city to city, upon 

 posts, about thirty feet high, and two hundred feet apart. The line 

 from Boston to New York is two hundred and thirty miles long. 

 The electricity goes to New York upon the upper wire, and returns 

 upon the lower one. To prevent the fluid from running away, the 

 wire? are wound around ci gla^s knob at every post, and, as elec- 

 tricity will not run over glass, it cannot escape. 



I will now explain another very important part of our subject ; 

 which is the 



ELECTRO-MAGNET. In the article on Magnetism, you will 

 find a description of the horse-shoe magnet ; but the electro- 

 magnet is a very different thing. A horse-shoe magnet is called a 

 natural magnet, because it has power of itself 

 to take up iron and steel. Now the electro- 

 magnet has no power of its own, being simply 

 a bar of iron, bent into the shape of a horse- 

 shoe magnet, and wound upon each side with 

 insulated wire, as you will see in the cut. Dur- 

 ing the passage of an electric current from the 

 battery along this wire, the bar exhibits a re- 

 markable degree of magnetic power, far superior to that of a steel 

 magnet of the same size. I have seen an electro-magnet, while 

 under the influence of a simple battery, sustain two fifty-six pound 



