their use is somewhat limited because one cannot change their
strength at will and this being the case, it is impossible to control
them. On the automobile we have permanent magnets in the speedometer
and ammeter as both of these instruments are measuring devices.
Electro Magnets
Electro magnets are made up of an iron core over
which is wound a number of turns of insulated wire as shown in
Fig. 269 When this device is connected with a source of electrical
energy, the core
Fig. 269— Electro Magnet
DIRECTION OF LINES OF FORCE— A
DIRECTION OF CURRENT FLOW
BATTERY
Fig. 270— Direction of Lines of Force
About a Current Carrying Conductor
134becomes magnetized and possesses all of the proper-ties found
in both natural and permanent magnets. To explain the reason for
this iron core becoming magnetized it will be necessary to show
the magnetic properties of the electric circuit.
Every current-carrying conductor has a magnetic field surrounding
it. This magnetic field always travels in a clockwise direction
around the conductor, looking in the direction in which the current
flows. In Fig. 270, we find the arrow indicates the direction of
current through the wire or conductor, and looking along the wire
from the end at which the current enters, we find the magnetic
field is traveling in a clockwise direction around it.
In order to determine the polarity of the electro magnet shown
in Fig. 271, the following rules are used:
1. Direct current flows through its circuit from positive to negative.
2. The magnetic field about a current-carrying conductor always
travels in a clockwise direction about the conductor, looking in
the direction in which current flows.
3. Lines of force always follow a conductor of magnetism where
they possibly can.
4. Lines of force always flow from the south to the north inside
the magnet and from north to south outside of the magnet.
By referring to Fig. 271, we find that the cur-rent flowing from
the positive of the battery passes around the core of the magnet
and back to the negative of the battery. The magnetic field surrounding
the wire is traveling clockwise around the wire and in so doing
engages the core while it is traveling upward and follows the iron
core. The lines of force emerging from the top of the magnet forms
a north pole and where they re-enter the core form the south pole.
Another method of checking the polarity of electro magnets is the
use of the "right hand rule" To do this, grasp the core
with the right hand with the fingers passing around the core in
the same direc- Fig. 27I— Polarity of Electro Magnet
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