Page:Popular Science Monthly Volume 92.djvu/485

 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

��469

�� �