Connect two copper wires to a voltaic cell and stretch a portion of the
wire over a compass needle, holding it parallel to it and as near as
possible without touching. Then bring the free ends of the wires
together and observe that the needle is deflected and after a few
movements back and forth comes to rest at an angle with the wire.
[Illustration: Fig. 76.—A Current of Electricity flowing through a Wire
will deflect a Compass Needle.]
Next form a rectangular loop of wire and place the needle within it as
in Figure 77. A greater deflection will now be obtained. If a loop of
several turns is formed, the deflection will be still greater.
These experiments were first performed by Oersted, in 1819, and show
that the region around a wire carrying a current of electricity has
_magnetic_ properties.
[Illustration: Fig. 77.—If a Loop of Wire is formed about a Compass
Needle, the Deflection will be greater.]
Another interesting experiment showing the magnetic effect of a current
of electricity when passing through a wire may be performed by
connecting a heavy copper wire to two or three bichromate-of-potash
cells. Dip the wire into a pile of fine iron filings and a thick cluster
of them will adhere to the wire as in Figure 78.
As soon as the circuit is broken so that the current of electricity
ceases flowing, the filings will fall off, showing that the magnetic
effect ceases with the current.
[Illustration: Fig. 78.—Iron Filings clustered on a Wire carrying a
Current of Electricity.]
These three simple experiments have shown that if a current of
electricity is passed through a copper wire, the wire will deflect a
compass needle, attract to itself iron filings, etc., as long as the
current continues to flow. As soon as the current is shut off, the
magnetic effect is _destroyed_.
The region in the neighborhood of a wire carrying a current is a _field
of force_ through which lines of magnetism are flowing in exactly the
same way that they do in the neighborhood of a bar or horseshoe magnet.
[Illustration: Fig. 79.—Magnetic Phantom formed about a Wire carrying a
Current of Electricity.]
This is readily shown by punching a small hole in a piece of cardboard,
and passing a wire carrying a strong current of electricity through the
hole.
If a few iron filings are sifted on the cardboard and the latter jarred
slightly with a pencil as they fall, they will arrange themselves in
circles with the wire at the center, forming a magnetic phantom and
showing the paths of the lines of magnetic force.
[Illustration: Fig. 80.—Magnetic Phantom formed about several Turns of
wire.]
By forming the wire into a coil as in Figure 80 the magnetic field
generated is much stronger and more plainly seen, for then the combined
effect of the wires is secured.
[Illustration: Fig. 81.—Paper Tube wrapped with Wire for Experimental
Purposes.]
Roll up a small paper tube about 1/2 inch in diameter and four inches
long. Wind neatly on the tube three layers of No. 18 insulated copper
wire. Pass an electric current through it from two or three cells of a
battery, and test its magnetic properties by bringing it near a compass
needle. It will be found that the coil possesses very marked magnetic
properties, and will readily cause the needle to swing about, even
though it is held quite a distance away.
If an iron bar is placed inside of the paper tube, the magnetic effect
will be greatly increased.
[Illustration: Fig. 82.—Showing how the Lines of Force "Leak" at the
sides of the coil, from a Coil of Wire, and how they are concentrated by
an Iron Core.]
The presence of the iron bar inside of the coil of wire greatly
increases the number of lines of force running through the coil.
[Illustration: Fig. 83.—The Principle of an Electro-Magnet.]
When a bar is not used, many of the lines of force leak out at the sides
of the coil, and but few extend from end to end. The effect of the iron
core is not only to diminish the leakage of the lines of force, but also
to add many more to those previously existing. Hence the magnetic
strength of a coil is greatly increased by the iron core.
A coil of wire wrapped around an iron core forms an _electro-magnet_.
[Illustration: Fig. 84.—if you wrap some insulated Wire around an
Ordinary Nail and connect it to a Battery, it will become an
Electro-Magnet.]
If you wrap some insulated wire around an ordinary nail and connect it
to one or two cells of a battery it will become an electro-magnet and
pick up bits of iron and steel.
If you wind the wire around a small paper tube into which a nail will
slide easily, the coil will draw the nail in when the current is turned
On. A hollow coil of this sort is called a solenoid.
Electro-magnets and solenoids play a part in the construction of almost
all electrical machinery. They form the essential parts of dynamos,
motors, telephone receivers, telegraph relays and sounders, and a host
of other devices.
The form usually given to an electro-magnet depends upon the use to
which it is to be put. The horseshoe is the most common. This consists
of two electro-magnets mounted on a yoke and connected so that the two
free poles are North and South.
[Illustration: Fig. 85.—If you wind the Wire around a small Paper Tube
into which a Nail will slide easily, the Coil will draw the Nail in when
the Current is turned on.]
Electro-magnets are made on a huge scale for lifting large castings and
heavy pieces of iron. Such magnets are used in the great steel mills and
in factories where nails, bolts, etc., are manufactured.
Monster electro-magnets can be seen in wonderful perfection at the great
steel mill at Gary, Indiana.
Ships bring the ore down the lakes to Gary, where great steel jaws lift
it out of the hold of the boat and carry it to the furnaces.
After being melted, great machines pour it out. It is divided into huge
ingots, and these, while hot, are carried to the first part of the
rolling mill.
The ingot is squeezed by a machine, made longer and narrower, then
squeezed again and made still longer and narrower.
It is started on its journey along the rollers of the mill, squeezed and
pressed here and there, as it travels hundreds of yards—no hand ever
touching it. It finally arrives, a red-hot steel rail, the right shape
and the right length.
During this time the steel has made a long journey and changed from a
shapeless ingot to a finished rail, handled entirely by machinery guided
and controlled by one or two operators, pressing levers and switches.
When almost finished, the rail slides down an incline before a man who
grasps the rail with huge pinchers, and standing at one end, runs his
eye along it. As he looks along the rail he sees the defects, moves the
left or the right hand, and another man at the levers of the
straightening machine, straightens the rail as directed.
And soon there are ten rails, perfectly straight, side by side, with
more coming down the incline to meet the glance of the man’s eye.
They are still too hot for any man’s touch and so a man sitting in a
tower touches an electric switch, and along the overhead rails there
comes gliding a monster electro-magnet.
The magnet moves along, drops down upon the ten rails, lying side by
side and weighing thousands of pounds. The man in the tower presses
another switch, thus turning on the current, and electricity glues the
rails to the magnet.
[Illustration: _By permission, from "Solenoids" by C. R. Underhill._
Lifting-Magnets of the type known as Plate, Billet, and Ingot Magnets.]
The ten rails are lifted at once, as easily as you would lift a needle
with your horseshoe magnet; they are carried to a flat-car, and when
lowered in position, the current is turned off, releasing the rails, and
the magnet travels back for another load.
Induction
In 1831, Michael Faraday, a famous English chemist and physicist,
discovered that if a magnet be suddenly plunged into a hollow coil of
wire, a momentary current of electricity is generated in the coil. As
long as the magnet remains motionless, it induces no current in the
coil, but when it is moved back and forth, it sets up the currents. The
source of electrical energy is the mechanical work done in moving the
magnet.
[Illustration: Fig. 86.—Showing how a Current of Electricity may be
induced by a Bar Magnet and a Coil.]
The medium which changes the mechanical energy into electricity is the
magnetic field which we have already seen exists in the neighborhood of
a magnet.
A current of electricity produced in a coil in such a manner is said to
be an _induced_ current and the phenomenon is that known as _magnetic
induction_.
Magnetic induction is met in the dynamo, induction coil, telephone,
transformer, some forms of motors, and a number of other electrical
devices.
[Illustration: Fig. 87.—A Horseshoe Magnet and a Coil arranged to
produce Electric Currents by _Induction_.]
A simple experiment in which electricity is produced by magnetic
induction may be performed by winding a number of turns of fine
insulated wire around the armature or keeper of a horseshoe magnet,
leaving the ends of the iron free to come in contact with the poles of
the permanent magnet. Connect the ends of the coil to a sensitive
galvanometer,¹ the ends of the armature being in contact with the poles
of the horseshoe magnet as shown in Figure 87.
Keeping the magnet fixed, suddenly pull off the armature. The
galvanometer will show a momentary current. Suddenly bring the armature
up to the poles of the magnet; another momentary current in the reverse
direction will flow through the circuit.
The fact that it is a reverse current is shown by the actions of the
galvanometer for it will be noticed that the needle swings in the
opposite direction this time.
It will also be noticed that no current is produced when the coil and
magnet are stationary. Current is only generated when the coil and
magnet are approaching one another or moving apart suddenly.
This is because it is only then that the magnetic field is changing. The
field is strongest nearest the magnet, and therefore if either the
magnet or the coil of wire is moved, the strength of that part of the
field which intersects the coil is changed. Induced currents can only be
generated by a _changing_ magnetic field.
¹ See chapter on Measuring Instruments.