<h2><SPAN name="CHAPTER_VII" id="CHAPTER_VII"></SPAN>CHAPTER VII.</h2>
<h3>ELECTRIC GENERATORS.</h3>
<p>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<span class="pagenum"><SPAN name="Page_63" id="Page_63"></SPAN></span>
another name for pressure; the symbol for
which is <i>E.M.F.</i></p>
<p>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.<span class="pagenum"><SPAN name="Page_64" id="Page_64"></SPAN></span>
So you see that metals are relative, not absolute,
in their character as positive and negative
elements.</p>
<p>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
Planté, 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.<span class="pagenum"><SPAN name="Page_65" id="Page_65"></SPAN></span>
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.</p>
<p>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<span class="pagenum"><SPAN name="Page_66" id="Page_66"></SPAN></span>
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.</p>
<p>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.</p>
<p>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<span class="pagenum"><SPAN name="Page_67" id="Page_67"></SPAN></span>
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.</p>
<p>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.</p>
<p>The dynamo is a machine that converts
mechanical into electrical energy, and the<span class="pagenum"><SPAN name="Page_68" id="Page_68"></SPAN></span>
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.</p>
<p>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<span class="pagenum"><SPAN name="Page_69" id="Page_69"></SPAN></span>
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.</p>
<p>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<span class="pagenum"><SPAN name="Page_70" id="Page_70"></SPAN></span>
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. (See
Chap. VI.)</p>
<p>Now mount the permanent magnet in a<span class="pagenum"><SPAN name="Page_71" id="Page_71"></SPAN></span>
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.</p>
<p>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.<span class="pagenum"><SPAN name="Page_72" id="Page_72"></SPAN></span>
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.</p>
<p>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 dy<span class="pagenum"><SPAN name="Page_73" id="Page_73"></SPAN></span>namo
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.</p>
<p>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.</p>
<p>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<span class="pagenum"><SPAN name="Page_74" id="Page_74"></SPAN></span>
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.</p>
<p>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.</p>
<p>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."</p>
<p>Any two metals having a difference of potential
will give the phenomena of thermo-electricity.
Antimony and bismuth having a<span class="pagenum"><SPAN name="Page_75" id="Page_75"></SPAN></span>
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.</p>
<p>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.</p>
<p>Some years ago, when the Postal Telegraph
Company first began operations between New<span class="pagenum"><SPAN name="Page_76" id="Page_76"></SPAN></span>
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.</p>
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