Page:Popular Science Monthly Volume 92.djvu/486

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��ropuiar bcience Monthly

��substance. While under the influence of the magnetic field, it behaves like a mag- net, and has polarity, the same as for a solenoid. A magnet, so produced, is termed an electromagnet; and the mag-

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���rig. 5

��Armature- rig. 6

��An iron bar encircled by a ciorrent is a core, and in a U-shape forms an electromagnet

netic substance (soft iron), around which the current circulates, is called the core — see Fig. 5. The magnetizing coil usually consists of a large number of turns of insulated wire.

Electromagnets differ from permanent magnets in several particulars: 1. They are made of soft iron instead of steel; 2. The magnetizing force is an electric current, and not another magnet; 3. The magnetic properties exist only while cur- rent flows in the magnetizing coil; 4. The magnetic strength is variable, approxi- mately proportional to the current flow- ing; 5. The polarity changes with change

��Armolure.

����A laminated core for an alternating cur- rent and the coils surrounding a core with lines of force about a magnetic circuit

in the direction of the current, and hence can be instantly reversed.

A typical form of electromagnet is illustrated in Fig. 6. On each of the legs of a horseshoe-shaped core is wound a cylindrical coil of insulated wire. The

��coils are so connected that current flows in opposite directions around the two legs of the magnet, making one end a north pole, and the other a south pole. When the soft iron armature is placed across the two poles, a closed iron circuit is ob- tained, and if the armature is large enough, most of the magnetic induction will be in the iron, since the lines of force will be closed curves. The number of lines of force produced in the core of an electromagnet may be considered as due to the relation of two factors, the mag- netizing power of the current in the mag- net coils, called the magnetomotive force, and the resistance to magnetization offered by the iron core, its reluctance, or

. - Magnetomotive force

Magnetic flux=zr-i — 7

Reluctance of core.

The magnetomotive force is produced by current circulating in the coil and so far as magnetism is concerned it does not matter whether 100 amperes of current flow once around the bar or whether one am- pere circulates 100 times. The mag- netizing force is always propor- tional to the prod- uct of number of turns and the cur- rent flowing in the

coil. This product is known as ampere turns.

The magnetic reluctance varies with the material used as core. It is prac- tically greatest with air and least with well annealed wrought iron. It also varies in inverse proportion to the cross- section of the core. The above is rigidly true for air and approximately true, within certain limits, for iron.

The cores for alternating current mag- nets must be laminated. A laminated core is made up of a number of thin plates, as shown in Fig. 7, The core is built in this way, as otherwise current would be induced in the iron and this cur- rent would heat the core and cause con- siderable waste of energy.

In designing electromagnets, it must always be borne in mind that the attrac- tion of an electromagnet for its armature varies as the square of the number of

���coil Fig 9

Conventional form of an electromagnet

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