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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