ELECTRIC GENERATORS.
Of the sources of electricity we have
mentioned two: Friction, and Galvanism or chemical
action. There are hundreds of forms of the latter
species of apparatus for generating electrical energy,
so we will mention only a few of the more prominent
ones. It is not our intention to go into the
chemistry of batteries. There are too many exhaustive
works on this subject lying on the shelves of libraries
that are accessible to all. All galvanic batteries
act on one general principle the generation
of electricity by the chemical action of acid on metal
plates; but the chemistry of their action is very different.
In all batteries the potential energy of one element
is greater than the other. The acid of the battery
dissolves the element of greater potentiality, and
its energy is freed and under right conditions takes
on the form of electricity. The potential of zinc,
for instance, is greater than that of copper, and
the measure of the difference is called the “electromotive
force,” the unit of which is the “volt.”
Electromotive force is another name for pressure; the
symbol for which is E.M.F.
If we were to put two zinc plates
in the battery fluid and connect them in the ordinary
way there would be no electricity evolved (assuming
that they were perfectly homogeneous), because they
are both of the same potential, or have the same possible
amount of stored electrical energy measured by its
working power. If one of the zinc plates were
softer than the other, a feeble current would be developed,
for one would be more readily acted upon by the acids
than the other. The battery that has been most
used in America for telegraphic purposes is called
the gravity-battery. It is constructed by putting
a copper plate in some form at the bottom of a jar,
usually of glass, and filling it partly full of the
crystals of sulphate of copper, commonly called “bluestone.”
Zinc, usually cast in some open form, so as to expose
a large surface to the solution, is suspended in the
upper part of the jar, which is then filled with water
till it covers the zinc. The zinc is the positive
metal, but it is called the negative pole. The
energy developed by the zinc passes from zinc to copper
and out on the circuit from the copper pole.
Hence the copper came to be called the positive pole,
although in relation to zinc it is negative.
Copper would, however, be positive to some other metal
whose potential was less. So you see that metals
are relative, not absolute, in their character as
positive and negative elements.
The galvanic battery has been almost
entirely superseded in this country for telegraphic
purposes by the dynamo, a machine developing electrical
currents by mechanical power. Another form of
battery that is extensively used for some kinds of
heavy current work is called the storage-battery.
The man who did the most, perhaps, to bring the storage-battery
to its present state of perfection was Plante, a Frenchman,
who died only a short time ago. Although very
many types of battery have been developed, it is found
that, after all, the lines on which he developed it
make the most efficient battery. There is a common
notion that electricity is stored in the storage-battery.
Energy is stored, that will produce electricity when
it is set free, just the same as energy is stored
in zinc. The storage-battery, when ready for action,
is one form of acid or primary battery. It has
been made by passing a current of electricity through
it until the chemical relations of the two lead plates
have been changed so that the potential of one is
greater than that of the other. A simple storage-battery
element is made up of two plates of lead held out
of contact with each other by some insulating substance
the same as the elements of an ordinary battery.
The cell is filled with dilute sulphuric acid, and
there will be no electrical action till the cell has
been charged by running a current of electricity through
it and forming a lead oxide on one plate. Now,
take off the charging battery and connect the two
poles, and electricity will flow until the oxide has
partly changed back into spongy metallic lead, when
it must be renewed by recharging.
I remember perfectly well the first
galvanic battery I ever saw, for it was of my own
construction. It is now nearly fifty years ago,
and yet it seems but yesterday such is
the flight of time. I related to you in another
chapter how I made a voltaic battery or
pile, as it was called by cutting up my
mother’s boiler and her stove-zinc, and the
domestic incident that followed. Well, a little
later I made a real galvanic battery as follows:
I lived in the country and far from town or city,
and my facilities were extremely limited, so that I
pursued my scientific investigations under great difficulties.
My only text-book was an old Comstock’s Philosophy.
In the book was a crude cut of a Morse register and
a short description of its construction, including
the battery. I determined to make a register,
and I did. It was all constructed of wood except
the magnet and its armature and the embossing-point,
which latter was made of the end of a nail. The
thing that seemed out of reach was the electromagnet.
I had no money; and there was no one that believed
I could do it, and if I could “what good would
come of it?” I made friends with a blacksmith
by keeping flies off a horse while he nailed the shoes
on, and “blowing the bellows” and occasionally
using the “sledge” for him. When I
thought the obligation had accumulated a sufficient
“voltage” (to express it electrically)
I communicated to the blacksmith the situation and
what I wanted.
The good-natured old fellow was not
long in bending up a U magnet of soft iron and forging
out an armature. The next step was to wind the
U with insulated wire. The only thing that I
had ever seen of the kind was an iron wire called
“bonnet” wire that was wrapped with cotton
thread. This, however, was not available, so
I captured a piece of brass bell-wire and wound strips
of cotton cloth around it for insulation and
in that way completed the magnet.
Now everything was ready but the battery.
I went at its construction with a feeling almost akin
to awe, for I could not believe that it would do as
described in the book. I procured a candy-jar
from the grocer and found some pieces of sheet zinc
and copper. These I rolled together into loose
spirals and placed one inside the other so that they
would not touch, when I was ready for the solution.
The druggist trusted me for a half pound of “blue
vitriol,” and I put it into my battery and filled
it with water. I waited awhile for it to dissolve,
and then connected my magnet in circuit, when to
my astonishment and delight it would lift
a pound or more. It was a great triumph.
I never have had one since that gave me the same satisfaction.
But I had my triumph all to myself. I was still
the same “tinker” (a name I had long carried),
and a nuisance to be endured but not encouraged.
The dynamo is the form of generator
now in general use where heavy currents of electricity
are needed. It is aptly described by a writer
in Modern Machinery, Mr. John A. Grier, as a thing
that when “at rest is a lifeless piece of mechanism;
in action it has a living spirit as full of mystery
as the soul of man.” This is a poetic way
of describing it that conveys to the mind a sense
of the power and beauty of natural law in action,
that would not come from a mere recital of the cold
scientific facts. The facts, however, are necessary:
but let us draw from them all the poetry and all the
practical lessons that we can as we go along; for
it is this blending of the poetic with the practical
that lends a charm to our every-day “grind,”
and lightens the load of many a weary hour.
The dynamo is a machine that converts
mechanical into electrical energy, and the great practical
value of energy in this form is that it can be distributed
through a conductor economically for many miles.
We can transmit mechanical power by means of a rope
or cable for a limited distance, but at tremendous
loss through friction. We can transmit power
through pipes by compressed air or steam, but there
is a great loss, especially in the case of steam,
by condensation from cold. None of these methods
are available for long distances. Another advantage
electricity has over other forms of energy is the speed
with which it can be transmitted from one place to
another. In this respect it has no rival except
light. But we have not been able to harness light
and make it available to carry either freight or news,
except in the latter case for a short distance by
flashing it in agreed signals.
The heliostat can be used when the
sun shines to transmit news by flashes of sunlight
chopped up into the Morse code and thrown from point
to point by a moving mirror. But this is limited
as to distance; besides, the sun does not always shine.
It has the disadvantage in that respect that the old
semaphore-telegraph did that was in use in Wellington’s
day. These semaphores were constructed in various
ways, but a common form was that of moving arms that
could be seen from hill to hill or point to point.
By a code of moving signals news was repeated from
point to point and it can be easily imagined that many
mistakes occurred, to say nothing of the time it required
for repetition. When the battle of Waterloo was
fought so the story goes news
was sent to England by means of the semaphore-telegraph.
The dispatch read, “Wellington defeated ”
At that point in the message a thick fog came up and
lasted for three days, so that no further news could
be sent or received. In the telegraphic parlance
of to-day the line was “busted.”
For three long days all London was in deep mourning,
when finally the fog lifted, which repaired the telegraphic
line, and the balance of the dispatch was received “the
French at Waterloo.” Mourning changed to
rejoicing and the English have rejoiced ever since
when they think of either Wellington or Waterloo.
But to return to the dynamo.
The name dynamo is an abbreviation for dynamo-electric
machine. A machine for producing dynamic electricity.
There are many forms of the dynamo, just as there are
in the evolution of every important machine, and there
will be many more. But the fundamental, underlying
principle of them all is contained in an experiment
made by Faraday. Faraday took the soft iron “keeper”
of a permanent magnet and wound insulated wire around
it and brought the two ends of the wire close together.
He now placed the keeper, with the wire wound around
it, across the poles of the permanent magnet, and
wrenched it away suddenly, when he observed a spark
pass between the ends of the wires. This would
occur when he approached the poles as well as when
he took it away. He discovered that the currents
were momentary and occurred at the moment of approach
or recession, and that the currents developed by the
approach were of opposite polarity to those occurring
at the recession. When the “keeper”
was put on the poles of the magnet it was magnetized
by having its molecular rings broken up and the poles
of the little natural magnets all turned in one direction.
During the time that the molecules of the keeper are
changing they are in a dynamic or moving condition.
By some mysterious action of the ether between the
iron and the wire wrapped around it there is a corresponding
molecular action in the wire that is dynamic for a
moment only, and during that moment we have the phenomenon
of an electric current. When the magnet and soft
iron are separated this molecular state of strain is
relieved and the molecules of both the iron and the
wire wound about it return to normal, and in the act
of returning we have a dynamic or moving condition,
resulting in a current, only in the opposite direction.
Now mount the permanent magnet in
a frame and mount the soft iron with the wire on it
(which in this shape is an electromagnet) on a revolving
arm and so set it on the arm that its ends will come
close to, but not touch, the poles of the permanent
magnet. Now revolve the arm, and every time the
electromagnet or keeper approaches the permanent magnet
a current of one polarity will be momentarily developed
in the wire of the electromagnet, which is moving.
When it is opposite the poles, it has reached the
maximum charge and, now, as it passes on it discharges
and a current of the opposite polarity is developed
in the wire. The more rapidly we revolve the
arm the more voltage (electrical pressure) the current
it develops will have.
It will be plain to all that we might
make the electromagnet stationary and revolve the
permanent magnet and get the same result. If the
permanent magnet were strong enough and the electromagnet
the right size as to iron, windings, etc., and
we revolve the arm with sufficient rapidity, we could
get an alternating current of electricity that would
produce an electric light. I have not and cannot
here give you the construction of a modern alternating-current
dynamo. I have simply described the simplest
form of dynamo, and all of them operate upon the fundamental
principle of a permanent magnetic field and an electromagnet,
moving in a certain relation to each other. The
field may revolve or the electromagnet may revolve,
whichever is the most convenient to construct.
The field-magnet may be a permanent magnet or an electromagnet,
made permanent during the operation of the dynamo by
a part of the current generated by the machine being
directed through a coil surrounding soft iron; or
the field-current may come from an outside source.
This is the kind of field-magnet universally used for
dynamo work, as a much stronger magnetism is developed
in this way than it is possible to obtain from any
system of permanent steel magnets.
The usual construction is to have
a stationary field-magnet and then a series of electromagnets
mounted and revolving upon a shaft in the center of
the magnetic field. The rotating part is called
the armature, and is so wound with insulated wire
that successive induced currents are created in the
armature windings and discharged through brushes which
rest on revolving segments that connect with the armature
windings. These induced currents succeed each
other with such rapidity as to amount in practice
to a steady current. However, the separate pulsations
are easily heard in any telephone when the circuit
is near to that of a dynamo circuit. The dynamo
current is not nearly so steady as the battery current,
although both are probably made up of separate discharges.
In the dynamo there is a discharge every time the
electromagnet of the armature cuts through the lines
of force of the magnetic field, and in the galvanic
battery every time a molecule is broken up and its
little measure of energy is set free. In the dynamo
the pulsations are so far apart as to make a musical
tone of not very high pitch, but in the galvanic battery
the pitch of the tone, if there is one, would require
a special ear to hear it one tuned, it may
be, up near the rate of light vibration.
There are two types of dynamo, one
generating a direct and the other an alternating current.
(By alternating we mean first a positive and then a
negative current impulse.) We cannot enter into a technical
description of the dynamo in a popular treatise such
as this.
The dynamo has evolved from the germ
discovered by Faraday, till to-day it is a machine,
the construction of which requires the highest class
of engineering skill. When in action it seems
like a great living presence, scattering its energy
in every direction in a way that is at once a marvel
and a blessing to mankind. But we must not give
all the credit to the dynamo. As the moon shines
with a reflected light, so the dynamo gives off energy
by a power delegated to it by the steam-engine that
rotates it, and the steam-engine owes its life to the
burning coal, and the burning coal is only giving
up an energy that was stored ages ago by the magic
of the sunbeam; and the sun ? Well, we
are getting close on to the borders of theology, and
being only scientists we had better stop with the
sun.
There is still another way of generating
electricity besides those that we have named; which
are friction, chemical action, and the magneto-electric
mode of generating a current. Electricity may
be generated by heat. If we connect antimony
and bismuth bars together and apply heat at the junction
of the metals and then connect the free ends of the
two bars to a galvanometer, it will indicate a current.
These pairs can be multiplied, and in this way increase
the voltage or pressure, and, of course, increase
the current, if we assume that there is resistance
in the circuit to be overcome. If there were absolutely
no resistance in the circuit a condition
we never find there would be no advantage
in adding on elements in series.
Substances differ in their resistance
to the passage of electricity the less
the resistance the better the conductor. The German
electrician, G. S. Ohm (1789-1854), investigated this
and propounded a law upon which the unit for resistances
is based, and this unit takes his name and is called
the “ohm.”
Any two metals having a difference
of potential will give the phenomena of thermo-electricity.
Antimony and bismuth having a great difference of
potential are commonly used. The use made of thermal
currents is chiefly for determining slight differences
of temperature. An apparatus called the thermo-electric
pile has been constructed out of a great number of
pairs of antimony and bismuth bars. This instrument
in connection with a galvanometer makes a most delicate
means of determining slight changes of temperature.
If one face of a thermopile is exposed to a temperature
greater than its own, the needle will move in one
direction; if to a temperature lower than its own,
the needle will be deflected in the opposite direction.
If both faces of the pile are exposed to the same
changes of temperature simultaneously, of course no
electrical manifestations will occur.
The earth is undoubtedly a great thermal
battery that is kept in action by the constant changes
of temperature going on at the earth’s surface,
caused by its rotation every twenty-four hours on its
axis. The sun, of course, is at some point heating
the earth, which at other points is cooling, making
a constant change of potential between different points.
If we heat a metal ring at one point a current of electricity
will flow around it especially if it is
made of two dissimilar metals until the
heat is equally distributed throughout the ring.
Some years ago, when the Postal Telegraph
Company first began operations between New York and
Chicago, the writer made observations twice a day
for some time of the temperature and direction of the
earth-current. The first two wires constructed
gave only two ohms resistance to the mile, which facilitated
the experiments. I found that in almost every
instance the current flowed from the point of higher
temperature to the lower. If the temperature
in New York were higher at the time of observations
than in Chicago the current would flow westward, and
if the conditions were reversed the current would
be reversed also.