414. =Electro-Magnets.=--The most powerful electro-magnets are made by bending a thick cylindrical bar of soft iron into the form of a horseshoe, A B, Fig. 286, and coiling around it a copper wire. The wire must be insulated by being wound with some non-conducting material, as silk, so that the electric current may pass through the whole length of the wire. With the instrument thus prepared, if the two ends of the wire be connected with the poles of a voltaic battery which is in action, the bar will be magnetized, and will hold up a heavy weight so long as the electric current is passing through the wire. Whenever the current is cut off by disconnecting the wires the weight will fall. [Illustration: Fig. 287.] Electro-magnets have been made in this way having such power as to sustain a weight of four thousand pounds. In Fig. 287 we have represented an apparatus which exhibits electro-magnetism very prettily. The soft iron, you see, is in two pieces which when put together form a ring, _d_, and each piece has a handle. If the pieces be put together with the coil, _c_, in the position represented, on connecting the wires P and N with a battery in action, the adhesion is so strong as to resist a great force; but as soon as the connection is broken the pieces come apart at once. [Illustration: Fig. 288.] 415. =Electric Telegraph.=--The most remarkable and useful application of electro-magnetism we have in the electric telegraph. As before stated, voltaic electricity is used. This is generated at the place from which the message is sent, and passes over the wire to the place where the message is received. There it acts upon soft iron by passing through coiled wire, producing the modified power called electro-magnetism. I will make all this plain to you by describing the machine used in Morse's Telegraph, Fig. 288. W W are the wires which connect with the station from which the message is to be received, and these connect with the copper wire coiled round the horseshoe of soft iron, _m m_. Above the magnet is a lever, _a l_, which works on a fulcrum at _d_. One end of this lever has a steel point, _s_, attached to it. At _c_ is an arrangement of wheel-work, the object of which is to pass along regularly a slip of paper, _p_, in the direction of the arrows. Observe now how the apparatus works. When the electric current passes through the coiled copper wire it makes a magnet of the iron, _m m_. The lever, _a l_, is therefore attracted at the end, _a_, downward. Of course the end, _l_, moves upward, bringing the steel point, _s_, against the paper, where it makes a mark. The length of this mark depends upon the length of time the electricity is allowed to pass along the coiled wire, for the moment that it is shut off _m m_ ceases to be magnetic, the "keeper," _a_, being no longer attracted, moves upward, and the other end, _l_, of the lever moves downward, taking the point, _s_, from the paper. [Illustration: Fig. 289.]