Electrical Devices and How They Work 



Principles of Electromagnets — III. 



It is the flow of current through a conductor wound 

 about a soft iron wire, that makes an electromagnet 



By Peter J. M. Clute, B. E. 



IT is evident that an electric current 

 and a magnet exert a mutual force on 

 each other. Since a magnetic field is 

 a region in which a magnetic needle is 

 acted upon by a force tending to turn it 

 in some direction, it follows that the space 

 surrounding a conductor, when an electric 

 current is flowing through it, is a magnetic 

 field. 



Knowing from experiments the direc- 

 tion of current in the conductor, the fol- 

 lowing rule is deduced for the direction 

 of the lines of force around the wire: 

 If you grasp the conductor with the 



Direction of 

 current 



Conductor 



Fig.l 



The lines of force will be around the con- 

 ductor in the direction shown by the coil 



right hand, with the extended thumb 

 pointing in the direction of the current, 

 the lines of force will be around the con- 

 ductor in the direction of the fingers, 

 as shown in Fig. 1. 



The direction of the lines of force 

 around a conductor is more clearly shown 

 by the arrows in Fig. 2, where it is as- 

 sumed that the current in the wire is 

 flowing toward the observer. Reversing 

 the direction of the current causes the 

 lines of force around the conductor to be 

 reversed. 



If a current-carrying conductor is bent 

 in the form of a loop, as in Fig. 3, all the 

 lines of force surrounding the conductor 

 pass through the loop in the same direc- 

 tion. Any magnetic substance placed 

 in front of the loop tends to place itself 

 with its longest axis projecting into the 

 loop, in the direction of the magnetic 

 force. 



By forming a helix of the conductor, 

 the lines of force around and inside each 

 loop will be similar, forming an equivalent 

 of long lines of force threading through 

 the entire helix. 



Wire 



The direction of 

 the lines of force are 

 shown by arrows 



The appearance 

 of the magnetic 

 field around a 

 helix through 

 which a current 

 is flowing, is il- 

 lustrated below 

 in Fig. 4. 



A helix con- 

 taining a number 

 of turns through 

 which current 

 flows is called a 

 solenoid. The 

 polarity of a sole- 

 noid, or the direction of the lines of force 

 through it, depends on the direction of the 

 current in the conductor. 



The polarity of a solenoid may be de- 

 termined by the following rule: Looking 

 at the end of the helix, if the current flows 

 around it clockwise, that end will be a 

 south pole; if in the other direction, it 

 will be a north pole. 



When a magnetic substance, such as 

 iron, is placed in a magnetic field, so that 



Direction of 

 force 



Showing the direction of the lines of force 

 in a loop or through a helix through 

 which a current of electricity is flowing 



the magnetic lines of force can reach it, 

 the substance immediately becomes mag- 

 netic. The lines of force appear to crowd 

 together and tend to pass through the 



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