Items related to Junior science
Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1921 Excerpt: ... 149.--An electromagnet made of a bar of soft iron surrounded by a coil of insulated wire carrying a current. The wire of the coils must be covered, or insulated; for if one turn of wire touches another, the current crosses over by a short cut instead of going through the full length of the wire. This is called "short circuiting" the current. It is important to have a large number of turns in the coil; for the more of them there are, the stronger the electromagnet which can be made. An interesting fact has been found to be true of the soft-iron core; a bundle of small wires makes a much stronger magnet than a single piece of iron having the size of the bundle. Could we use an electromagnet to sweep up the iron chips and turnings of a factory? 231. How Do We Ring an Electric Bell?--Have you ever seen one of the old-fashioned house bells which you rang by pulling a knob? There was a wire attached to the knob, and there was a spring attached to the other end of the wire. When you pulled the knob, you set the spring in motion, and the moving spring jingled the bell. The modern doorbell can be put in almost any out-of-the-way place, and its wire can go around corners and through walls. One thing it must have, however, which the older bell did not need: a battery, or some other source of an electric current. Examine an electric bell, and compare it with the diagram in Fig. 150. Push Button Trace the circuit from the battery to the push button. Here the circuit is broken, and no current can pass to the electromagnet. But if we press the push button against the little metal triangle shown below it, we close the circuit. Then the current passes through the coil, and causes the core to become a magnet. The core now draws to itself the piece of iron (the arm...
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