470 



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- 



YoKe- 



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 



