Page:Popular Science Monthly Volume 92.djvu/487

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��lines of force passing through both, and it should then be endeavored to obtain the maximum flux that a current can pro- duce; that is, to arrange a circuit with the least possible magnetic resistance.

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General form of a large iron-clad lifting- magnet used with a crane or lift for a hoist

As a general rule, it will be found ad- visable to make the thickness of the coil about equal to that of the core; to make the yoke just long enough so that the coils will not interfere with each other when placed in position ; and to make the core long enough to accommodate the necessary wire. In all coil winding proper insulation must be provided to prevent a short circuit. The kind of insulation de- pends on the size and use of a magnet.

In Fig. 8 is shown a cross-section of an electromagnet, showing the coils sur- rounding the core and a general scheme of the lines of force existing in such a mag- netic circuit. The armature is shown out of contact with the magnet and con- siderable leakage flux is depicted. If the armature is brought into contact with the core, it will not only reduce this leakage, but it will increase the flux of the magnet by lowering the magnetic re- luctance of the circuit.

A few of the principal forms of electro- magnets, including the horseshoe magnet, the electro-bar magnet, and the iron-clad magnet are shown in Fig. 6, 9 and 10.

��The Sleeper Must Get Up to Stop the Alarm

THE growing habit of switching off the alarm clock and then sleeping a while longer made one commuter miss his train many times. He very easily overcame this difficulty, however, by making an attachment to his bed springs, as shown in the illustration, that com- pelled him to get up to break the electric current.

A simple frame was made of two brass spring pieces, stretched lengthwise of the bed, one on each side and just under the springs, with a third piece connecting them about 2 ft. from the head. These wires were properly insulated from the bed frame with pieces of fiber attached in the line as shown at A and B. A small turnbuckle C was used to keep the line taut.

A cheap clock made the electric con- nection at the time set, and a couple of dry batteries completed the device. The connections were simple, one wire was attached from the battery, through the bell to the bed springs, the other from the battery, through the clock to the in-

���The weight of the sleeper makes the con- tact for the circuit on the bed springs

��sulated frame. The clock turned on the current at the time set. The weight of the sleeper caused the springs to touch the frame, as shown by the dotted line, so the bell rang and kept on ringing until

he got up. — J. K. BURRELL.

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