ELECTRIC CURRENTS.
The simplest form of an electric machine is one in which the operator is
a prominent part of the operation. Electricity, like magnetism, operates
in a closed circuit, even when it is static--so-called. Take a stick of
sealing-wax, say, in your left hand, and rub it with a piece of fur or
silk with your right hand, and you have the simplest form of electric
machine--the one that was known to the ancients, and the one from which
the science, great as it is to-day, had its beginnings. The stick of
sealing-wax is one element of the battery, and the piece of fur or silk
is the other, while your hands, arm and body form the conductor that
connects the two poles, and the friction is the exciting agent and may
be said to take the place of the fluid of a battery. The electrical
conditions are not wholly static, as a slow current is passing around
through your arms and body from one pole to the other. Even if the
conditions were wholly static there would be polarized lines of force,
in a state of strain, reaching around in a closed circuit.
If we rub the wax with the fur and then take it away the wax has a
charge of electricity and will attract light objects. If we had rubbed a
piece of metal or some good conductor it would have been warmed instead
of electrified. In both cases the particles of the substances have been
affected, and if the atomic theory is correct--and it seems
plausible--in the former case the atoms are partly put into electrical
motion and partly into a state of electrical strain that we call static
(standing) electricity; while in the latter case the atoms are put into
the peculiar motion that belongs to heat. The former we call
electricity, and the latter we call heat. The electro-atomic motion
under some circumstances readily turns to heat, which seems to be the
tendency of all forms of energy. The electric light is a result of this
tendency. All non-conductors, or electrics, have a complex molecular
structure, and, while their atoms when subjected to friction are put
into a state of electrostatic strain, they are not able readily to
respond as a conductor of dynamic electricity. The electric-light
filament in the incandescent lamp is a much poorer conductor than the
copper wire that leads up to it. The copper wire is readily responsive
to the electrical influence, but the carbon filament is not. So
electrical action that freely passes along the wire, is resisted and
becomes heat action in the filament, and light is the attendant of
intense heat. But, to go back to the sources of electricity.
Frictional electric machines have been constructed in great variety.
All, however, embrace the essentials set forth in the sealing-wax
experiment, and would be difficult to describe without cuts. Let us,
therefore, consider another source of electricity, which was the
outgrowth of the discovery of Galvani (or rather his wife), and reduced
to concrete form by Volta. We refer to the galvanic or voltaic battery.
If we put a bar of zinc into a glass vessel and pour sulphuric acid and
water into it, there will be a boiling, and an evolution of hydrogen
gas, and energy is released in the form of heat, so that the fluid and
the glass vessel become heated. Now let us put a bar of copper or a
stick of carbon into the glass, but not in contact with the zinc;
connect the ends (that are not immersed) of the two elements--copper and
zinc--with a metal wire or any conductor, and a new condition is set up.
Heat is no longer evolved to the same extent, but most of the energy
becomes electrical in character, and an electrical chain of action takes
place in the circuit that has now been formed. Taking the zinc as the
starting point, the so-called current flows from the zinc through the
fluid to the copper and from the copper through the wire to the zinc.
A chain of polarized atomic activity is established in the circuit,
similar to the closed circuit of magnetic lines of force, only the
latter is static, while the former is dynamic.
You ask what is the difference? Well, it is much easier to ask a
question than it is to answer it. You will remember that in the chapter
on magnetism it was stated that the molecules of a magnet were little
natural magnets, and that their attractions were satisfied within
themselves; that when their local attachments were broken up and all
their like poles turned in one direction they could act upon other
pieces of iron outside of the magnet. Outside and between the poles
there are magnetic lines of force reaching out from one pole to the
other. If we put a piece of iron across the poles these lines of force
are gathered up and pass through the iron. This is purely a static
condition. Let us go back to the cell of battery. When the elements are
in position (the copper, the acidulated water and the zinc), and the two
wires attached to the two metals which are the two poles of the battery
not yet connected, there is a condition induced in these two wires that
did not exist before the acidulated water was poured in, although the
circuit is not yet established. If we test the two wires we find a
difference of potential--a state of strain, so to speak--that did not
exist before the acid acted on the zinc and liberated what was stored
energy. It is in a static condition, like the magnet, and electrical
lines of force are reaching out from both wires so that the ether is in
a state of strain between the two poles. The air molecules may partake
of it, but we have to bring in the ether as a substance, because the
same conditions would practically exist if the two wires were in a
vacuum. If now we connect the two wires, we have established a metallic
circuit between the two poles of the battery, the static conditions are
relieved, the lines of force are gathered up into the wire, and the
phenomenon that we call a current is established and we have dynamic or
moving electricity.
Having established the so-called electric current we will now try to
show you that there really is no current. The idea of a current involves
the idea of a fluid substance flowing from one point to another. When
you were a boy did you never set up a row of bricks on their ends, just
far enough apart so that if you pushed one over they all fell one after
another? Now, imagine rows of molecules or atoms, and in your
imagination they may be arranged like the bricks, so that they are
affected one by the other successively with a rapidity that is akin to
that of light-waves, and you can conceive how a motion may be
communicated from end to end of a wire hundreds of miles in length in a
small fraction of a second, and no material substance has been carried
through the wire--only energy. We do not mean to say that the row of
bricks illustrates the exact mode of molecular or atomic motion that
takes place in a conductor. What we mean is, that in some way motion is
passed along from atom to atom.
To give you a better conception of an electric current, let us go back
of the galvanic cell to the electric machine. If both poles of the
machine are attached to rods terminating in round knobs we can set the
machine in action and keep up a steady stream of disruptive discharges
that will, if their frequency is great enough, perform the function of a
current, and we have dynamic electricity from a statical machine; when
the acid of the galvanic battery breaks down a molecule of zinc, energy
is set free, and in the battery we have what corresponds to a disruptive
discharge of infinitesimal proportions. This discharge would have been
immediately converted into heat energy if the copper element had been
left out of the battery, but as it is, it impresses itself on the atomic
"brick" next to it, which establishes a chain of atomic movement
throughout the circuit. This may constitute, if you please, a line of
electrical force. But as thousands of these disruptive discharges are
taking place simultaneously as many different lines of force are
established. You must not conceive of these chains of atoms as simply
thrown down like the bricks and left lying there, but that the atom is
active; that it has the power to pick itself up again in an
infinitesimally short time and is again knocked down (following the
illustration of the bricks) by the next discharge along its line or
chain of atoms.
If you could get a mental picture of this action you would see that the
whole conductor is in a most violent state of atomic motion of a
peculiar kind. At the same time a part of this electrical motion is
being converted into a heat motion of the atoms, and finally it all
returns to heat unless some of it is stored up somewhere as potential
energy. If the current has driven a motor that has wound up a weight, a
part is stored up in the weight, which has the ability to do work if it
is allowed to run down. If it drives machinery as it runs down, the
mechanical motion is the expression of the stored energy. When the
weight has run down the energy will be represented by the heat created
by friction of the journals of the wheels and pulleys and the heating of
the air. If the weight is allowed to fall suddenly it will heat the air
to some extent, but mostly the earth and the weight itself will be
heated. If the source of energy (the battery) is great and the pressure
high and the conductor is too small to carry the energy developed in the
battery as electricity, heat is developed, and if the heat is
sufficiently intense, light also.
We have seen (Vol. II) that heat motion when it reaches a sufficiently
high rate throws the ether into a vibratory motion that we call light.
However, this vibratory motion of the ether is set up long before it
reaches the luminous stage; in other words, there are dark rays of the
ether. We find that the electro-atomic motions of a conductor have the
power to impress themselves upon the ether.
[Illustration: Fig. 1.
A is the primary line; _a_, the battery: _b_, the key. B is the
secondary line in which is placed the galvanometer _c_.]
Let us try another experiment to show that this is the case, not only,
but that the impressed ether can transfer these impressions to still
another conductor. Suppose we stretch two parallel wires for, say, half
a mile, or any distance, only a few feet apart, and make of each a
complete circuit by rounding the end of the course and returning the
wire to the starting point (as shown in Fig. 1). Put in one of these
circuits a battery, and a circuit-breaker (a common telegraph-key), and
in the other circuit a galvanometer (an instrument for detecting the
presence and measuring the intensity of a galvanic current, by means of
a dial and a deflecting needle or pointer). Now if we touch the key and
close the circuit in A, the needle of the galvanometer in B will swing
in one direction from zero on the dial; and if we release the key,
breaking the circuit in A, the needle will swing back in the opposite
direction. In neither case will the needle stay deflected, but will at
once return to zero.
This shows that when the battery current was allowed to complete its
circuit through wire A by closing its key, an electrical action was
instantly felt in wire B, although there was no material connection
between them other than the air, which is a non-conductor.
The current in the second circuit is called an induced current. Why this
current? According to one theory, when we close the primary circuit the
surrounding ether is thrown into a peculiar state of strain that we will
call magnetic or electrical lines of force. When the ether wave strikes
the second wire there is a molecular movement from a state of rest to a
state of static strain. During the time that the molecules are moving
from the normal to the strained position in sympathy with the ether we
have the condition of a dynamic current, which lasts only a moment. This
state of strain continues till the circuit is opened (breaking the
wire-line), when all the electrical lines of force vanish and the
molecular strain of the second wire is relieved, and we again have the
conditions, momentarily, for a current of the opposite polarity, and the
needle will swing in the opposite direction because the molecules or
atoms have, in their recoil to the natural state, moved in an opposite
direction.
Going back to Fig. 1, let us further study the phenomena under other
conditions. In our first circuit (A) there is a battery and a
circuit-breaker, which is a common telegraph-key. Now close the key so
that a current will be established. (Remember that "current" is only a
name for a condition of dynamic charge.) Place a piece of soft iron
across the wire at right angles with the direction of the wire, when of
course it will be at right angles with the direction of the current, and
you will find now that the iron is more or less magnetic, depending upon
the amount of current passing through the wire. If we wind a number of
turns of insulated wire through which the current is passing around the
iron the magnetism will be increased. In practice there are a certain
number of turns and a certain sized wire that will give the best results
with a given number of cells of battery (or a given voltage or
pressure), operating in a closed circuit of a given resistance. All
these questions are worked out mathematically in many standard books on
the subject. It is not the intention in these talks to develop the
science mathematically but to set out the fundamental physical facts and
applications of electricity.
Under the conditions above named magnetism is developed in the soft iron
bar. If we open the key the current will cease and the magnetism will
vanish--that is to say, the molecules will turn back to their neutral
position by their own attractions, as has been described in a previous
chapter. Magnetism developed in this way is called electromagnetism.
(See Chap. IV.) If we use a piece of hardened steel instead of the soft
iron it will become magnetic and remain so when the circuit is opened,
because the natural tendency of the molecules to turn back to the
neutral position is not great enough to overcome the coercive force, or
molecular friction, of hardened steel, as has been also described in a
previous chapter. To make the best electromagnet we need qualities of
iron just the opposite from those of the permanent magnet. For the
former we need the purest of soft iron, well annealed (heated to redness
and slowly cooled, making it less brittle), so that its molecules are
free to turn; while for the latter we need hardened steel, so that when
the molecules are once wrenched into the magnetic condition they cannot,
of themselves, turn back to the neutral state. The great value of the
electromagnet lies in its ability to readily discharge, or go back to
the neutral state, when the current is broken.
Let us now go back to the beginning of our experiment. When we closed
the key and established the current through the wire we found that a
piece of iron held at right angles to the wire, although not touching
it, became magnetic. We have already said that when the circuit was
open, the battery being in circuit, there were electrical lines of force
established in the ether, between the two poles of the battery, and that
they were gathered up into the conducting wire when the circuit was
closed. We now find that there are other lines of force of a different
nature established in the ether when the circuit is closed. These we
call magnetic lines of force, or the magnetic field of the charged wire,
and they are established at right angles to the direction of the
current. These magnetic lines of force acting through the ether from an
electrically charged conductor are able to break up the natural
molecular magnetic rings, referred to in Chapter IV, and turn all their
like poles in the same direction--thus making one compound magnet of the
iron which in the neutral state consisted of millions of little natural
magnets whose attractions were satisfied by a joining of their unlike
poles.
Most writers account for all of the phenomena of induced currents in a
second wire as coming directly from these magnetic lines of force
developed upon closing the circuit.
So much for theory based upon a set of facts that make the theory seem
probable. If you don't like it give us a better one. If it is correct
the writer claims no credit; it is merely a compilation of suggestions
from many sources, including his own experience. We are simply seeking
after truth. The man who is an earnest seeker after scientific truth
cannot afford to pursue his investigations with any prejudice in favor
of one theory more than another, unless the facts sustain him, and then
he is not acting from prejudice, but is led by the facts. Many people
make pets of their theories; and they become attached to them as they do
their children; and they look upon a man who destroys them by a
presentation of the facts as an enemy. I once knew a lady who became so
attached to her family doctor that, she said, she would rather die under
his treatment, if necessary, than to be cured by any other doctor. There
are many people who are imbued with this kind of spirit not only in
matters scientific, but in matters religious as well. Such people are
not the kind who contribute to the world's progress, but are the
hindrances that have to be overcome.