THE ELECTRIC TELEGRAPH. 



establishment of the current, and their loss of it upon the suspen- 

 sion of the current, are, for all practical purposes, instantaneous. 

 The moment the extremities of the wire coiled round the horse- 

 shoe are put into connection with the poles of the battery the 

 horse-shoe becomes a magnet, and the moment the connection with 

 the battery is broken it loses the magnetic virtue. 



146. It has been already shown, that by means of very simple 

 expedients, the current may be interrupted hundreds or even 

 thousands of times in a second, being fully re-established in the 

 intervals. The acquisition and loss of magnetism by the horse- 

 shoe accompany these pulsations with the most perfect and 

 absolute simultaneity. If the pulsations of the current be pro- 

 duced, at the rate of a thousand per second, the alternate presence 

 and absence of the magnetic virtue in the horse-shoe will equally 

 be produced at the rate of a thousand per second. Nor are these 

 effects in any way modified by the distance of the place of inter- 

 ruption of the current from the magnet. Thus, pulsations of the 

 current may be produced by an operator in London, and the 

 simultaneous pulsations of the magnetism may take place at 

 Vienna, provided only that the two places are connected by a con- 

 tinuous series of conducting wires. 



147. It remains to show how these rapid pulsations of the mag- 

 netism of the bar can be rendered sensible, and how they may 

 even be estimated and counted. 



Let two straight rods of soft iron be surrounded by a succession 

 of convolutions of covered wire, such as has been already de- 

 scribed, and let the ends, m, m', fig. 58, of these rods be connected 

 Fi 5g by a straight bar of soft iron, attached 



to them by screws and nuts. Let the 

 wire, a b } proceeding from a distant 

 station, s, be put in metallic connection 

 with the extremity of the wire coiled 

 upon the rod, m, and let the wire, a' b', 

 connected with the extremity of the 

 last convolution of the wire on the rod, 

 m', be put in metallic connection with 

 the earth. If a current flow along a b, 

 it will therefore circulate round the rods, 

 m and m', and will pass to the earth by 

 the wire, a' b'. So long as this current 

 flows, the rods will be magnetic, and 

 they will lose their magnetism in the 

 intervals of its suspension. 

 Let g h be a light iron bar, supported on a pivot, at o, on which 

 it is capable of playing, so that its arm, o g, may move freely to 

 198 



