Page:Popular Science Monthly Volume 20.djvu/696

676 I can raise the poker from its position without difficulty; but, now I join on the wires connected with the battery to the coil of the electromagnet, the poker is attracted with greater force than my strength can overcome. The effect is more perceptible by the use of nails instead of the poker. When the current is on you see that, owing to the attraction of the nails by the magnet, I am able to build up an ornamental triumphal arch of nails, but, directly I take off the current, the magnetism ceases, and the nails succumb to the force of gravity, and fall to the floor. I have here the parts of an electro-magnet disjointed, which I will put together to illustrate the utility of this power of attraction. I place the iron bar surrounded with wire on the table, and, on holding nails to it, you see it possesses no attractive power. If I let the current pass from the battery through the coil, the iron is magnetized, the attractive action is immediately set up, and the nails are attracted. Now, suppose I want to utilize this power to ring this bell—the dome which I struck a short time ago—I place the hammer (which is attached to a piece of iron in position to be attracted similar to the nails, so as to imitate the motion of my wrist) in front of the electro-magnet; I then place the bell-dome on its rod, so that the hammer, when drawn forward, will fall upon it; and, as you will see, the arrangement is complete in that simple form, for each time a current is sent the iron armature is attracted by the magnet bringing with it the hammer which strikes the bell by its forward motion. This is the principle on which all electric bells are constructed. A similar arrangement is employed for giving alarms in case of burglary. A small switch arrangement is attached to the frame-work of the bedroom-door [a small door so fitted was exhibited]; this switch is put on on going to bed, and, should the door be opened by any one during the night, a succession of currents of electricity are sent to a bell, which continues to ring till attended to, and it would be a bold burglar who dared to carry on his nefarious business under such a din. A modification of this arrangement can be adapted to innumerable purposes. In America, where servants are not so plentiful as in England, necessity has compelled the employment of electricity for domestic purposes, to a far greater extent than in England. In nearly every town in America, systems of wires extend to almost every house, in the hall of which is a small instrument [one was exhibited], on the dial of which divisions bear the various words—messenger, carriage, coupé, express-wagon, doctor, police, fire, and two or three other things, and on turning the instrument pointer to any division, and drawing down a spring, a call is sent to the central office or bureau, and the instrument there indicates the service required and at what house, and the request is carried out at once. While traveling in America, I was once afraid that I should not have time to reach the nearest telegraph-office, to dispatch a telegram in reference to an appointment; and, on mentioning my concern to the lady in whose