ELECTRICAL APPARATUS.
What is electricity?--Forms of electricity--Magnetism--The
permanent magnet--Lines of force--Electro-magnets--The electric
bell--The induction coil--The condenser--Transformation of
current--Uses of the induction coil.
WHAT IS ELECTRICITY?
Of the ultimate nature of electricity, as of that of heat and light, we
are at present ignorant. But it has been clearly established that all
three phenomena are but manifestations of the energy pervading the
universe. By means of suitable apparatus one form can be converted into
another form. The heat of fuel burnt in a boiler furnace develops
mechanical energy in the engine which the boiler feeds with steam. The
engine revolves a dynamo, and the electric current thereby generated can
be passed through wires to produce mechanical motion, heat, or light. We
must remain content, therefore, with assuming that electricity is energy
or motion transmitted through the ether from molecule to molecule, or
from atom to atom, of matter. Scientific investigation has taught us how
to produce it at will, how to harness it to our uses, and how to measure
it; but not _what_ it is. That question may, perhaps, remain unanswered
till the end of human history. A great difficulty attending the
explanation of electrical action is this--that, except in one or two
cases, no comparison can be established between it and the operation of
gases and fluids. When dealing with the steam-engine, any ordinary
intelligence soon grasps the principles which govern the use of steam in
cylinders or turbines. The diagrams show, it is hoped, quite plainly
"how it works." But electricity is elusive, invisible; and the greatest
authorities cannot say what goes on at the poles of a magnet or on the
surface of an electrified body. Even the existence of "negative" and
"positive" electricity is problematical. However, we see the effects,
and we know that if one thing is done another thing happens; so that we
are at least able to use terms which, while convenient, are not at
present controverted by scientific progress.
FORMS OF ELECTRICITY.
Rub a vulcanite rod and hold one end near some tiny pieces of paper.
They fly to it, stick to it for a time, and then fall off. The rod was
electrified--that is, its surface was affected in such a way as to be in
a state of molecular strain which the contact of the paper fragments
alleviated. By rubbing large surfaces and collecting the electricity in
suitable receivers the strain can be made to relieve itself in the form
of a violent discharge accompanied by a bright flash. This form of
electricity is known as _static_.
Next, place a copper plate and a zinc plate into a jar full of diluted
sulphuric acid. If a wire be attached to them a current of electricity
is said to _flow_ along the wire. We must not, however, imagine that
anything actually moves along inside the wire, as water, steam, or air,
passes through a pipe. Professor Trowbridge says,[11] "No other agency
for transmitting power can be stopped by such slight obstacles as
electricity. A thin sheet of paper placed across a tube conveying
compressed air would be instantly ruptured. It would take a wall of
steel at least an inch thick to stand the pressure of steam which is
driving a 10,000 horse-power engine. A thin layer of dirt beneath the
wheels of an electric car can prevent the current which propels the car
from passing to the rail, and then back to the power-house." There
would, indeed, be a puncture of the paper if the current had a
sufficient voltage, or pressure; yet the fact remains that _current_
electricity can be very easily confined to its conductor by means of
some insulating or nonconducting envelope.
MAGNETISM.
The most familiar form of electricity is that known as magnetism. When a
bar of steel or iron is magnetized, it is supposed that the molecules in
it turn and arrange themselves with all their north-seeking poles
towards the one end of the bar, and their south-seeking poles towards
the other. If the bar is balanced freely on a pivot, it comes to rest
pointing north and south; for, the earth being a huge magnet, its north
pole attracts all the north-seeking poles of the molecules, and its
south poles the south-seeking poles. (The north-_seeking_ pole of a
magnet is marked N., though it is in reality the _south_ pole; for
unlike poles are mutually attractive, and like poles repellent.)
There are two forms of magnet--_permanent_ and _temporary_. If steel is
magnetized, it remains so; but soft iron loses practically all its
magnetism as soon as the cause of magnetization is withdrawn. This is
what we should expect; for steel is more closely compacted than iron,
and the molecules therefore would be able to turn about more easily.[12]
It is fortunate for us that this is so, since on the rapid magnetization
and demagnetization of soft iron depends the action of many of our
electrical mechanisms.
THE PERMANENT MAGNET.
Magnets are either (1) straight, in which case they are called bar
magnets; or (2) of horseshoe form, as in Figs. 50 and 51. By bending the
magnet the two poles are brought close together, and the attraction of
both may be exercised simultaneously on a bar of steel or iron.
LINES OF FORCE.
In Fig. 50 are seen a number of dotted lines. These are called _lines of
magnetic force_. If you lay a sheet of paper on a horseshoe magnet and
sprinkle it with iron dust, you will at once notice how the particles
arrange themselves in curves similar in shape to those shown in the
illustration. It is supposed (it cannot be _proved_) that magnetic force
streams away from the N. pole and describes a circular course through
the air back to the S. pole. The same remark applies to the bar magnet.
ELECTRICAL MAGNETS.
[Illustration: FIG. 50.--Permanent magnet, and the "lines of force"
emanating from it.]
If an insulated wire is wound round and round a steel or iron bar from
end to end, and has its ends connected to the terminals of an electric
battery, current rotates round the bar, and the bar is magnetized. By
increasing the strength and volume of the current, and multiplying the
number of turns of wire, the attractive force of the magnet is
increased. Now disconnect the wires from the battery. If of iron, the
magnet at once loses its attractive force; but if of steel, it retains
it in part. Instead of a simple horseshoe-shaped bar, two shorter bars
riveted into a plate are generally used for electromagnets of this type.
Coils of wire are wound round each bar, and connected so as to form one
continuous whole; but the wire of one coil is wound in the direction
opposite to that of the other. The free end of each goes to a battery
terminal.
In Fig. 51 you will notice that some of the "lines of force" are
deflected through the iron bar A. They pass more easily through iron
than through air; and will choose iron by preference. The attraction
exercised by a magnet on iron may be due to the effort of the lines of
force to shorten their paths. It is evident that the closer A comes to
the poles of the magnet the less will be the distance to be travelled
from one pole to the bar, along it, and back to the other pole.
[Illustration: FIG. 51.--Electro-magnet: A, armature; B, battery.]
Having now considered electricity in three of its forms--static,
current, and rotatory--we will pass to some of its applications.
THE ELECTRIC BELL.
A fit device to begin with is the Electric Bell, which has so largely
replaced wire-pulled bells. These last cause a great deal of trouble
sometimes, since if a wire snaps it may be necessary to take up carpets
and floor-boards to put things right. Their installation is not simple,
for at every corner must be put a crank to alter the direction of the
pull, and the cranks mean increased friction. But when electric wires
have once been properly installed, there should be no need for touching
them for an indefinite period. They can be taken round as many corners
as you wish without losing any of their conductivity, and be placed
wherever is most convenient for examination. One bell may serve a large
number of rooms if an _indicator_ be used to show where the call was
made from, by a card appearing in one of a number of small windows.
Before answering a call, the attendant presses in a button to return the
card to its normal position.
In Fig. 52 we have a diagrammatic view of an electric bell and current.
When the bell-push is pressed in, current flows from the battery to
terminal T^1, round the electro-magnet M, through the pillar P and
flat steel springs S and B, through the platinum-pointed screw, and back
to the battery through the push. The circulation of current magnetizes
M, which attracts the iron armature A attached to the spring S, and
draws the hammer H towards the gong. Just before the stroke occurs, the
spring B leaves the tip of the screw, and the circuit is broken, so that
the magnet no longer attracts. H is carried by its momentum against the
gong, and is withdrawn by the spring, until B once more makes contact,
and the magnet is re-excited. The hammer vibrations recur many times a
second as long as the push is pressed in.
[Illustration: FIG. 52.--Sketch of an electric-bell circuit.]
The electric bell is used for so many purposes that they cannot all be
noted. It plays an especially important part in telephonic installations
to draw the attention of the subscribers, forms an item in automatic
fire and burglar alarms, and is a necessary adjunct of railway
signalling cabins.
THE INDUCTION OR RUHMKORFF COIL.
Reference was made in connection with the electrical ignition of
internal-combustion engines (p. 101) to the _induction coil_. This is a
device for increasing the _voltage_, or pressure, of a current. The
two-cell accumulator carried in a motor car gives a voltage (otherwise
called electro-motive force = E.M.F.) of 4·4 volts. If you attach a wire
to one terminal of the accumulator and brush the loose end rapidly
across the other terminal, you will notice that a bright spark passes
between the wire and the terminal. In reality there are two sparks, one
when they touch, and another when they separate, but they occur so
closely together that the eye cannot separate the two impressions. A
spark of this kind would not be sufficiently hot to ignite a charge in a
motor cylinder, and a spark from the induction coil is therefore used.
[Illustration: FIG. 53.--Sketch of an induction coil.]
We give a sketch of the induction coil in Fig. 53. It consists of a core
of soft iron wires round which is wound a layer of coarse insulated
wire, denoted by the thick line. One end of the winding of this
_primary_ coil is attached to the battery, the other to the base of a
hammer, H, vibrating between the end of the core and a screw, S, passing
through an upright, T, connected with the other terminal of the battery.
The action of the hammer is precisely the same as that of the armature
of an electric bell. Outside the primary coil are wound many turns of a
much finer wire completely insulated from the primary coil. The ends of
this _secondary_ coil are attached to the objects (in the case of a
motor car, the insulated wire of the sparking-plug and a wire projecting
from its outer iron casing) between which a spark has to pass. As soon
as H touches S the circuit is completed. The core becomes a powerful
magnet with external lines of force passing from one pole to the other
over and among the turns of the secondary coil. H is almost
instantaneously attracted by the core, and the break occurs. The lines
of force now (at least so it is supposed) sink into the core, cutting
through the turns of the "secondary," and causing a powerful current to
flow through them. The greater the number of turns, the greater the
number of times the lines of force are cut, and the stronger is the
current. If sufficiently intense, it jumps any gap in the secondary
circuit, heating the intermediate air to a state of incandescence.
THE CONDENSER.
The sudden parting of H and S would produce strong sparking across the
gap between them if it were not for the condenser, which consists of a
number of tinfoil sheets separated by layers of paraffined paper. All
the "odd" sheets are connected with T, all the "even" with T^1. Now,
the more rapid the extinction of magnetism in the core after "break" of
the primary circuit, the more rapidly will the lines of force collapse,
and the more intense will be the induced current in the secondary coil.
The condenser diminishes the period of extinction very greatly, while
lengthening the period of magnetization after the "make" of the primary
current, and so decreasing the strength of the reverse current.
TRANSFORMATION OF CURRENT.
The difference in the voltage of the primary and secondary currents
depends on the length of the windings. If there are 100 turns of wire in
the primary, and 100,000 turns in the secondary, the voltage will be
increased 1,000 times; so that a 4-volt current is "stepped up" to 4,000
volts. In the largest induction coils the secondary winding absorbs
200-300 miles of wire, and the spark given may be anything up to four
feet in length. Such a spark would pierce a glass plate two inches
thick.
It must not be supposed that an induction coil increases the _amount_ of
current given off by a battery. It merely increases its pressure at the
expense of its volume--stores up its energy, as it were, until there is
enough to do what a low-tension flow could not effect. A fair comparison
would be to picture the energy of the low-tension current as the
momentum of a number of small pebbles thrown in succession at a door,
say 100 a minute. If you went on pelting the door for hours you might
make no impression on it, but if you could knead every 100 pebbles into
a single stone, and throw these stones one per minute, you would soon
break the door in.
Any intermittent current can be transformed as regards its intensity.
You may either increase its pressure while decreasing its rate of flow,
or _amperage_; or decrease its pressure and increase its flow. In the
case that we have considered, a continuous battery current is rendered
intermittent by a mechanical contrivance. But if the current comes from
an "alternating" dynamo--that is, is already intermittent--the
contact-breaker is not needed. There will be more to say about
transformation of current in later paragraphs.
USES OF THE INDUCTION COIL.
The induction coil is used--(1.) For passing currents through glass
tubes almost exhausted of air or containing highly rarefied gases. The
luminous effects of these "Geissler" tubes are very beautiful. (2.) For
producing the now famous X or Röntgen rays. These rays accompany the
light rays given off at the negative terminal (cathode) of a vacuum
tube, and are invisible to the eye unless caught on a fluorescent
screen, which reduces their rate of vibration sufficiently for the eye
to be sensitive to them. The Röntgen rays have the peculiar property of
penetrating many substances quite opaque to light, such as metals,
stone, wood, etc., and as a consequence have proved of great use to the
surgeon in localizing or determining the nature of an internal injury.
They also have a deterrent effect upon cancerous growths. (3.) In
wireless telegraphy, to cause powerful electric oscillations in the
ether. (4.) On motor cars, for igniting the cylinder charges. (5.) For
electrical massage of the body.
[11] "What is Electricity?" p. 46.
[12] If a magnetized bar be heated to white heat and tapped with a
hammer it loses its magnetism, because the distance between the
molecules has increased, and the molecules can easily return to their
original positions.