THE PRODUCTION OF MAGNETISM BY ELECTRICITY
[Illustration: FIG. 15.--Diagram to illustrate Magnetic effect of an
Electric Current.]
In the previous chapter attention was drawn to the fact that there
are many close parallels between electric and magnetic phenomena, and
in this chapter it will be shown that magnetism can be produced by
electricity. In the year 1819 Professor Oersted, of the University of
Copenhagen, discovered that a freely swinging magnetized needle, such
as a compass needle, was deflected by a current of electricity flowing
through a wire. In Fig. 15, A, a magnetic needle is shown at rest in
its usual north and south direction, and over it is held a copper wire,
also pointing north and south. A current of electricity is now sent
through the wire, and the needle is at once deflected, Fig. 15, B. The
direction of the current is indicated by an arrow, and the direction
in which the needle has moved is shown by the two small arrows. If the
direction of the current is reversed, the needle will be deflected in
the opposite direction. From this experiment we see that the current
has brought magnetic influences into play, or in other words has
produced magnetism. If iron filings are brought near the wire while the
current is flowing, they are at once attracted and cling to the wire,
but as soon as the current is stopped they drop off. This shows us that
the wire itself becomes a magnet during the passage of the current, and
that it loses its magnetism when the current ceases to flow.
[Illustration: FIG. 16.--Magnetic Field round wire conveying a Current.]
Further, it can be shown that two freely moving parallel wires
conveying currents attract or repel one another according to the
direction of the currents. If both currents are flowing in the same
direction the wires attract one another, but if the currents flow in
opposite directions the wires repel each other. Fig. 16 shows the
direction of the lines of force of a wire conveying a current and
passed through a horizontal piece of cardboard covered with a thin
layer of iron filings; and from this figure it is evident that the
passage of the current produces what we may call magnetic whirls round
the wire.
A spiral of insulated wire through which a current is flowing shows
all the properties of a magnet, and if free to move it comes to rest
pointing north and south. It is attracted or repelled by an ordinary
magnet according to the pole presented to it and the direction of the
current, and two such spirals show mutual attraction and repulsion.
A spiral of this kind is called a _solenoid_, and in addition to the
properties already mentioned it has the peculiar power of drawing
or sucking into its interior a rod of iron. Solenoids have various
practical applications, and in later chapters we shall refer to them
again.
If several turns of cotton-covered wire are wound round an iron rod,
the passing of a current through the wire makes the rod into a magnet
(Plate II. _b_), but the magnetism disappears as soon as the current
ceases to flow. A magnet made by the passage of an electric current
is called an _electro-magnet_, and it has all the properties of the
magnets mentioned in the previous chapter. A bar of steel may be
magnetized in the same way, but unlike the iron rod it retains its
magnetism after the current is interrupted. This provides us with
a means of magnetizing a piece of steel much more strongly than is
possible by rubbing with another magnet. Steel magnets, which retain
their magnetism, are called _permanent_ magnets, as distinguished from
electro-magnets in which soft iron is used, so that their magnetism
lasts only as long as the current flows.
Electro-magnets play an extremely important part in the harnessing
of electricity; in fact they are used in one form or another in
almost every kind of electrical mechanism. In later chapters many of
these uses will be described, and here we will mention only the use
of electro-magnets for lifting purposes. In large engineering works
powerful electro-magnets, suspended from some sort of travelling
crane, are most useful for picking up and carrying about heavy masses
of metal, such as large castings. No time is lost in attaching the
casting to the crane; the magnet picks it up directly the current
is switched on, and lets it go the instant the current is stopped.
In any large steel works the amount of scrap material produced is
astonishingly great, hundreds of tons of turnings and similar scrap
accumulating in a very short time. A huge mound of turnings is
awkward to deal with by ordinary manual labour, but a combination of
electro-magnet and crane solves the difficulty completely, lifting
and loading the scrap into carts or trucks at considerable speed, and
without requiring much attention.
Some time ago a disastrous fire occurred at an engineering works in the
Midlands, the place being almost entirely burnt out. Amongst the débris
was, of course, a large amount of metal, and as this was too valuable
to be wasted, an electro-magnet was set to work on the wreckage. The
larger pieces of metal were picked up in the ordinary way, and then the
remaining rubbish was shovelled against the face of the magnet, which
held on to the metal but dropped everything else, and in this way some
tons of metal were recovered.
The effect produced upon a magnetized needle by a current of
electricity affords a simple means of detecting the existence of such
a current. An ordinary pocket compass can be made to show the presence
of a moderate current, but for the detection of extremely small
currents a much more sensitive apparatus is employed. This is called
a _galvanometer_, and in its simplest form it consists essentially of
a delicately poised magnetic needle placed in the middle of a coil of
several turns of wire. The current thus passes many times round the
needle, and this has the effect of greatly increasing the deflection
of the needle, and hence the sensitiveness of the instrument. Although
such an arrangement is generally called a galvanometer, it is really
a galvanoscope, for it does not measure the current but only shows its
presence.
We have seen that electro-motive force is measured in volts, and that
the definition of a volt is that electro-motive force which will cause
a current of one ampere to flow through a conductor having a resistance
of one ohm. If we make a galvanometer with a long coil of very thin
wire having a high resistance, the amount of current that will flow
through it will be proportionate to the electro-motive force. Such a
galvanometer, fitted with a carefully graduated scale, in this way
will indicate the number of volts, and it is called a _voltmeter_.
If we have a galvanometer with a short coil of very thick wire, the
resistance put in the way of the current is so small that it may be
left out of account, and by means of a graduated scale the number of
amperes may be shown; such an instrument being called an _amperemeter_,
or _ammeter_.
For making exact measurements of electric currents the instruments just
described are not suitable, as they are not sufficiently accurate; but
their working shows the principle upon which currents are measured. The
actual instruments used in electrical engineering and in scientific
work are unfortunately too complicated to be described here.