THE ELECTRIC TELEGRAPH. 



extended from m to n, or of mo if it were as described above in 

 connection with, the earth at o, were anywhere broken, no current 

 would be produced by the entrance or withdrawal of the magnet. 

 It is therefore essential to the production of these phenomena that 

 the extremities m and n of the coil wire shall be in electric com- 

 munication with each other, by being united either with a 

 continuous metallic connection, or by means of the earth in the 

 manner already described. 



The property in virtue of which soft iron acquires magnetic 

 properties, when the poles of a permanent magnet are brought 

 into proximity with it, supplies a very convenient method of 

 exhibiting the play of the phenomena of momentary currents above 

 described. 



162. Let SON (fig. 62), be a powerful permanent horse-shoe 

 magnet, having its poles s, N, presented to and in close proximity 

 with a similar horse- shoe a b of soft iron, 

 wrapped with convolutions of covered wire 

 in the manner already described. Let the 

 extremities m and n of the coil be sup- 

 posed to be placed in connection with two 

 wires, which may be extended to any dis- 

 tances, and whose extremities are in me- 

 \ n tallic communication with the earth in the 

 manner already explained. 



When the poles s and N are brought into 

 proximity with the ends a and b of the 

 horse-shoe a b, the latter will, by the 

 inductive action of the magnet s o N, ac- 

 quire magnetic polarity, the end a, near the south pole s, having 

 northern, and the end b, near the north pole N, having southern 

 polarity. This magnetic polarity, however, of a b will only con- 

 tinue so long as the poles s and sr of the permanent magnet are 

 kept near to a and b. If they be removed, that instant the 

 polarity of a b will cease. If the poles be reversed, N being pre- 

 sented to a, and s to 6, then a will acquire south, and b north 

 polarity. 



It appears, therefore, that by presenting the poles of the magnet 

 N o s to the horse-shoe the same effect is produced as if the poles 

 of a magnet were suddenly passed into the axis of the coil, and by 

 withdrawing the poles sr and s from a and &, the same effect is 

 produced on the coil as if the poles of the magnet which had been 

 passed along the axis were suddenly withdrawn. 



208 



