The Londonderry sifter. (South Londonderry, Vt.) 1883-19??, 1891-05-21
ELECTRIC DYNAMOS.
MACHINERY THAT CAUSES THE Al
TERNATlNG CURRENT.

Comparison Betweon the First Frictional
Wheel and the Present Powerful Eiec-
tro- Magncts--D Difference In the Two
Currents-The Machine.
The first dynamo electric machine
ever constructed was made by Faraday,
This great physicist, the prince Of ex.
perimenters, as he has been called, dis
covered that when disc or fat plate Of
copper was made to rotate between the
poles of a powerful magnet currents were
produced in the plate from the center out
ward. By making wire touch the re-
volving plate with one of its ends and
bringing the other one in contact with
the rim he found that current of cleo
tricity passed along the wire, and could
be made to indicate its existence by de-
flecting the needle of galvanometer,
decomposing chemical solution, o, by
any of the well known effects prod iced
by electricity in motion.
Faraday saw the importance of this
discovery and the great uses in the way
of practical application to which it
might be put, but be did not himself
stay to develop it; he left that to Others,
and with it the wealth which might
thus be acquired, and himself went on
to investigate other obscure and little
known phenomena connected with phys
les and electricity, regarding this as his
proper work, and exhibiting in his con-
duct the true scientific spirit. When
many years afterward he went to see
the first application of this discovery of
his to the production of the illumina-
ton of the North Foreland lighthouse,
he said, after looking at the large mag
neto-electric machines there, 81 gave 1t
to you an infant; you have made it a
giant"
Dynamo and magnetic electric ma
chimes consist essentially of coil of wire
-"the armature," as it is called-rotat-
ing between the poles of large mag
net, the poles being bent round so as to
approach each other and have the arma-
ture between them. This magnet may
be either permanent magnet of hard
steel, or an eirctro-magIet consisting of
wire cooled round a soft iron core, a cur
rent of electricity being made to circle
round the wire coil, and thus magnetic
ing the iron core while it lasts. 1t is the
latter arrangement which is almost uni
versally used now, though the magnetic
machines with permanent magnets were
the earliest form.
THE ELECTRO-MAGNET.
A magnet produces an influence in the
neighborhood around it, and this sur
rounding neighborhood is known as the
''field of force" of the magnet-i. e., the
sphere in which its influence can be felt
A magnetic needle or bit of iron filing
placed in this field sets itself to point
along the Zines of force" of the field
that is, tho lines along which the mag
nctic force acts, and which form curves
round the magnet, running out, as it
were, from pole to pole, and curving
round to the other. Any one may see
the form of these lines of force for him
self by placing bar magnet underneath
sheet of paper and then sprinkling
filings on the paper.
On tapping this the filings will set
themselves along the lines of force in
beautiful regular curves. Here the small
fragments Of iron are themselves made
magnet while under the influence of the
powerful magnet in whose "tield" they
are, and therefore place themselves
lens,thwise along the lines of force,-tbat
is, along the line of action of the result
ant magnetic force at the place where
each one is.
When coil of wire or armature is
made to revolve rapidly in the strong
field of force which occupies the. space
between the poles of powerful electro
magnet currents are produced in the
coil. These currents alter their direc-
ton through thu coil every time the lat
ter changes its position with reference
to the poles Of the magnet The side of
the coil, which was opposite the north
pole, is after half revolution opposite
the south pole, and the influence of the
south pole tends to produce an opposite
current to that of the north pole. Here
We have an alternate current" dynamo
machine.
PROCESSES OF USING THE CURRENT.
as the coil or armature rotateS with
great speed-solre hundreds of revolu-
tons per minute-these currents, in al
ternating directions, succeed each other
very rapidly, and if an electric arc lamp
is placed on the circuit it will be lit up.
In this case it IS not necessary that the
current be sent round the circuit in one
direction only, but although the termi-
nalsfof the lamp are constantly changing
their polarity-that is, the north pole
where the current enters the next in
slant becomes the south pole where the
current leaves-yet, as this occurs many
times In one second, the effect produced
is the same as if the current was in one
uniform direction.
The lamp has no time to cool; it does
not go out before the oppositely directed
current passes through it and produces
the same effect as the previous one. No
Pickering is observable. The impression
produced BY the glowing carbon on the
human eye is retained by the retina for
far longer period than the duration of
one surge of electricity through the
lamp, and is not gone before the effect
produced by the succeeding opposite
wave makes its impression on our nerves.
In a continuous current" dynamo,
which is necessary for some purposes,
such as electro-plating, where the effect
desired could not be produced if the di-
rection of the current was continually
altering, the electric current is made to
pass always one way round the external
circuit. This result is got by using the
ingenious device of a commutator.
which automatically defects the current
SO as always to send it in an unvarying
direction through the plating bath or
the electric lamp, as the case may be-
knowledge.
LET'S and Rights.