Техническое чтение для энергетиков. Бухарова Г.П. - 53 стр.

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55
MAGNETIC EFFECT OF AN ELECTRIC CURRENT
The invention of the voltaic cell in 1800 gave electrical experimenters a source
of a constant flow of current. Seven years later the Danish scientist and experimenter,
Oersted, decided to establish the relation between a flow of current and a magnetic
needle. It took him at least 13 years - more to find out that a compass needle is
deflected when brought near a wire through which the electric current flows. At last,
during a lecture he adjusted, by chance, the wire parallel to the needle. Then, both he
and his class saw that when the current was turned on, the needle deflected almost at
right angles towards the conductor. As soon as the direction of the current was
reversed, the direction the needle pointed in was reversed too.
As seen the north end of the needle moves away from us when the current flows
from left to right. Oersted also pointed out that provided the wire were adjusted
below the needle, the deflection was reversed.
The above-mentioned phenomenon highly interested Ampere who repeated the
experiment and added a number of valuable observations and statements. He began
his research under the influence of Oersted's discovery and carried it on throughout
the rest of his life.
Everyone knows the rule thanks to which we can always find the direction of the
magnetic effect of the current. It is known as Ampere's rule. Ampere established and
proved that magnetic effects could be produced without any magnets by means of
electricity alone. He turned his attention to the behaviour of the electric current in a
single straight conductor and in a conductor that is formed Into a coil, i.e. a solenoid.
When a wire conducting a current is formed into a coil of several turns, the
amount of magnetism is greatly increased.
It is not difficult to understand that the greater the number of turns of wire, the
greater is the m.m.f. (that is I he magnetomotive force) produced within the coil by
any constant amount of current flowing through it. In addition, when doubling the
current, we double the magnetism generated in the coil.
A solenoid has two poles which attract and repel the |poles of other magnets.
While suspended, it takes up a north and a south direction exactly like the compass
needle.
A core of iron becomes strongly magnetized if placed within the solenoid while
the current is flowing.
When winding a coil of wire on an iron core, we obtain an electromagnet. That
the electromagnet is a controllable and reliable magnet is perhaps known to everyone.
It is, so to say, a temporary magnet provided by electricity. Its behaviour is very
simple. The device is lifeless unless an electric current flows through the coil.
However, the device comes to life provided the current flows. The iron core will act
as a magnet as long as the current continues to pass along the winding.
THE DISCOVERY OF ELECTRO-MAGNETIC INDUCTION
It is at this important juncture in the history of electrical research that we see

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