_What is electro-magnetism?_
It is the magnetism developed through the agency of _electrical_ or
_galvanic action_.
_What were the earliest phenomena observed which indicated a
relation between magnetism and electricity?_
It was noticed that _ships’ compasses_ have their directive power
impaired by lightning, and that sewing needles could be rendered
magnetic by electric discharges passed through them.
_What discovery, made by Prof. Oersted of Copenhagen, established
beyond a doubt the connection of electricity and magnetism?_
He ascertained that a magnetic needle placed near a metallic wire
connecting the poles of a galvanic battery was compelled to change its
direction, and that the new direction it assumed was determined by its
position in _relation to the wire_ and to the direction of the current
_transmitted along the wire_.
Thus, if a needle be inclosed in a wire not touching it at any point,
and a current of electricity pass through the wire, the needle will be
made to move in accordance with the direction of the current.
_What other important discovery was made about the same time?_
It was found that if a piece of soft iron, not possessing magnetic power
sufficient to elevate a grain weight, be placed within a coil of copper
wire through which a galvanic current is passing, it will become,
through the influence of the current, a _powerful magnet_; and will, so
long as the current flows, sustain weights amounting to many hundreds of
pounds.
_Is the magnetic power of the bar found to be wholly dependent on
the existence of the current?_
It _is_; the moment the current stops, the weights _fall away_ from the
bar in obedience to the law of gravity.
_How great weights have been lifted by magnets formed in this
manner?_
An electro-magnet constructed by Prof. Henry was capable of elevating
and sustaining about a _ton weight_.
_Upon what principle does the construction of the Morse magnetic
telegraph depend?_
Upon the principle that a current of _electricity_ circulating about a
bar of soft iron is capable of _rendering it a magnet_.
_Why is it necessary, in conveying the telegraph wires, to support
them upon glass or earthen cylinders?_
These are used for the purpose of insuring the perfect _insulation_ of
the wires, since but for this the electricity would pass down a damp
pole to the earth, and be lost.
_Is there any truth in the idea that many persons have, that some
principle passes along the telegraphic wires when intelligence is
transmitted?_
This supposition is _wholly erroneous_; the word current, as something
flowing, conveys a false idea, but we have no other term to express
electrical progression.
_How can we gain an idea of what really takes place, and of the
nature of the influence transmitted?_
The earth and all matter are _reservoirs of electricity_; if we disturb
this electricity at Boston by voltaic influence, its pulsations may be
felt in Chicago. Suppose the telegraphic wire were a tube, extending
from Boston to Chicago, filled with water. Now, if one drop more is
forced into it at Boston, a drop must fall out at Chicago, but no drop
was caused to pass from Boston to Chicago. Something similar to this
occurs in the transmission of electricity.
_What was the earliest important industrial application of
electricity?_
One of the earliest industrial applications of electricity was to the
driving of street cars. The first electric railway was installed by
Siemens, of Berlin, in 1882; and the system was quickly taken up and
brought to a high state of development by American engineers. It is
remarkable that the system of traction early adopted is the one
generally used in America and Europe until the present date. It consists
essentially of (_a_) a supply of _continuous current_ at five hundred to
five hundred and fifty volts, generated in (_b_) a _central powerhouse_,
and transmitted to the car by means (_c_) of _overhead conductors_,
whence by contact with a trolley wheel on a pole on the car it is led
down to (_d_) _two series-excited motors_, which are placed electrically
first _in series_ with one another _at starting_, and then _in parallel_
with one another when a sufficient speed has been attained.
_To what well-known electrical machines did this give impetus?_
Electric _dynamos_ and motors. All such machines will convert the energy
of mechanical motion into that of electricity in motion, or the reverse.
The former conversion is done by _dynamos_, to which power is given by
steam-engines or other such prime-movers, and made to generate in
conducting circuits alternate or direct currents of electricity.
_Motors_, on the other hand, receive the energy of electrical currents,
either alternate or direct, and this produces motion of certain parts of
the structure.
The theory of the action of a dynamo was first discovered by Faraday in
1831; it is intimately associated with that of a motor, for the
principle of conservation of energy points out that either machine is
reversible--that is to say, a dynamo may be used as a motor or a motor
as a dynamo, though perhaps not so efficiently as when each fulfills the
special function for which it was designed.
THE CURRENT IN A DYNAMO OR MOTOR.--This brings us to the production of
an electric current by the dynamo. In the dynamo we have a coil of wire
moving across a magnetic field, alternately passing into this field and
out of it. A magnetic field is produced, as we have just seen, by the
steady movement of electrons, and we may picture it as being a region of
the ether disturbed or strained by the effect of the moving electrons.
When the coil of wire passes into the magnetic field, the electrons of
its atoms are influenced powerfully and set in motion in one direction,
so producing a current in the coil. As the coil passes away from the
field, its electrons receive a second impetus, which checks their
movement and starts them traveling in the opposite direction, and
another current is produced. The coil moves continuously and regularly,
passing into and out of the magnetic field without interruption; and so
we get a current which reverses its direction at regular intervals, that
is, an _alternating_ current.
[Illustration: =THE TELEGRAPH AND ITS WONDERFUL INSTRUMENTS=