NIAGARA FALLS POWER--APPLIANCES.
In the last chapter I described some of the appliances used in
connection with the power-house. There are many things that are
commonplace as electrical appliances when used with currents of low
voltage and small quantity, that become extremely interesting when
constructed for the purpose of handling such currents as are developed
by the dynamos used at Niagara. For instance, it is a very commonplace
and simple thing to break and close a circuit carrying such a current as
is used for ordinary telegraphic purposes, but it requires quite a
complicated and scientifically constructed device to handle currents of
large volume and great pressure. If such a current as is generated by a
dynamo giving out 5000 horse-power under a pressure of 2200 volts should
be broken at a single point in a conductor, there would be a flash and a
report, attended with such a degree of heat and such power for
disintegration that it would destroy the instrument.
The circuit-breakers used at Niagara are constructed with a very large
number of contacts made of metal sleeves, or tubes, say one inch in
diameter, so constructed that one will slide within the other; the
sleeves being slotted so as to give them a little spring that secures a
firm contact. These are all connected together electrically, on each
half of the switch, as one conductor, so that when the switch is closed
the current is divided into as many parts as there are points of contact
in the switch. Suppose there are 100 of these contact-points, a
one-hundredth part of the current would be flowing through each one of
them. If, now, these points are so arranged that they can be all
simultaneously separated, the spark that will occur at each break will
be very small as compared with what it would be if the whole current
were flowing through a single point, and it would be so small that there
would be no danger attending the opening of the switch. These switches
are carefully guarded, being boxed in and under the control of a single
individual.
There is another apparatus that is a necessary part of every
manufacturing or other kind of plant that uses electricity from this
power-house, and this is called the transformer. Many of you are
familiar with the box-shaped apparatus that is used in connection with
electric lighting when the alternating current is used. Where simply
heating effects are required, such as in electric lighting, for
instance, the alternating current can be used to greater advantage than
the direct current when it has to be carried to some distance, owing to
the fact that it may be a current of high voltage. A greater amount can
be carried through a small conductor; thus greatly reducing the cost of
an electrical plant that distributes power to a distance. A transformer
is an apparatus that changes the current from one voltage to another.
In the ordinary electric-light plant, such as is used in a small town or
village, the current that is sent out from the power-station has a
pressure of from 1000 to 1500 volts, according to the distance to which
it is sent. It would not do, however, for the current to enter a
dwelling at this high pressure, because it is dangerous to handle, and
the liability to fires originating from the current would be greatly
increased. At some point, therefore, outside of the building, and not a
great distance from it, a transformer is inserted which changes the
voltage, say, from 1000 down to 50 or 100, according to the kind of
lamps used. Some lamps are constructed to be used with a current of
fifty volts and others for 100 or more. The lamp must always be adapted
to the current or the current to the lamp, as you choose. The human body
may be placed in a circuit where such low voltage is used without
danger, but it would be exceedingly dangerous to be put in contact with
a pressure of 1000 or more volts, such as is used for lighting purposes.
In principle the transformer is nothing more or less than an
induction-coil on a very large scale. The ordinary induction-coil, such
as is used for medical purposes, is ordinarily constructed by winding a
coarse wire around an iron core. This core is usually made of a bundle
of soft iron wires, because the wires more readily magnetize and
demagnetize than a solid iron core would. Around this coil of coarse
wire, which we call the primary coil, is wound a secondary coil of finer
wire. If now a battery is connected with the primary coil, which is made
of the coarse wire, and the circuit is interrupted by some sort of
mechanical circuit-breaker, each time the primary or battery circuit is
opened there will be a momentary impulse in the secondary circuit of a
much higher voltage; and at the moment the primary circuit is closed
there will be another impulse in this secondary circuit in the opposite
direction. The latter impulse is called the initial and the former the
terminal impulse. A current created in this manner is called an
_induced_ current. The initial current is not so strong as the terminal
in this particular arrangement.
If we should take hold of the two wires connected with the two poles of
the battery and bring them together so as to close the circuit, and then
separate them so as to break it we should scarcely feel any
sensation--if there were only one or two cells, such as are ordinarily
used with such coils. But if we connect these wires to the coils of the
induction apparatus and then take hold of the two ends of the secondary
coil and break and close the primary circuit we should feel a painful
shock at each break and close, although the actual amount of current
flowing through the secondary wire is not as great as that which flows
through the primary; but the voltage (or electromotive force) is higher,
and thus is able to drive what current there is through a conductor of
higher resistance, such as the human body. For this reason there is more
current forced through the body, which is a poor conductor, than can be
by a direct battery current which has a lower voltage. If now we should
take a battery of a number of cells, so as to get a voltage equal to
that given off by the secondary coil, and connect it with the fine-wire
coil instead of the coarse-wire coil--thus making what was before the
secondary coil the primary--by breaking and closing the battery circuit
as before we shall get a secondary or induced current in the coarse-wire
coil, but it will be a current of low voltage, and will not produce the
painful sensation that the secondary coil did.
We have now described the principle of a transformer as it is worked out
in an ordinary induction-coil. As has been stated, at Niagara Falls the
current comes from the dynamos with an electromotive force or pressure
of 2200 volts. For some purposes this voltage is not high enough, and
for other purposes it is too high; therefore it has to be transformed
before it is used! For some purposes this transformation takes place in
the power-house, and for others it takes place at the establishment
where it is used. For instance, take the current that is sent to
Buffalo, a distance of from twenty to thirty miles. The current first
runs to a transformer connected with the power-house, where it is
"stepped-up" (to use the parlance of the craft) from a voltage of 2200
to 10,000. It is carried to Buffalo through wire conductors that are
strung on poles, and is there "stepped-down" again through another
transformer to the voltage required for use at that place. The object of
raising the voltage from 2200 to 10,000 in this case is to save money in
the construction of the line of conductors between the two points. If
the voltage were left at 2200--the conductors remaining the same as they
are now--the loss in transmission would be very great, owing to the
resistance which these wires would offer to a current of such
comparatively low voltage as 2200. To overcome this difficulty--if the
voltage is not increased--it would be necessary to use conductors that
are very much larger in cross-section (thicker) than the present ones
are. And as these conductors are made of copper the expense would be too
great to admit of any profit to the company.
If we go back to an illustration we used in one of the early chapters on
electricity we can better explain what takes place by increasing the
voltage. If we have a column of water kept at a level say of ten feet
above a hole where it discharges, that is one inch in diameter, a
certain definite amount of water will discharge there each minute. If
now we substitute for the hole that is one inch in diameter one that is
only one-half inch in diameter a very much smaller amount of water will
discharge each minute, if the head is kept at the same point--namely,
ten feet. But if now we raise the column of water we shall in time reach
a height which will produce a pressure that will cause as much water to
discharge per minute through the one-half-inch hole as before discharged
through the one-inch hole with only the pressure of a ten-foot column.
This is exactly what takes place when the voltage is "stepped-up," which
is equivalent to an increase of pressure.
It will be seen from the foregoing that these transformers have to be
made with reference to the use the current is to be put to. In general
shape they are alike in appearance, the difference being chiefly in the
relation the primary sustains to the secondary coils. There is another
kind of transformer that is used when it is necessary to have the
current always running in the same direction. This transformer, as
heretofore explained, does not change the voltage of the current, but
simply transforms what was an alternating into a direct current. By
alternating current we mean one that is made up of impulses of
alternating polarity--first a positive and then a negative. The direct
current is one whose impulses are all of one polarity. The direct
current is required for all purposes where electrolysis (chemical
decomposition by electricity, as of silver for silver-plating, etc.) is
a part of the process. The alternating current may be used without
transformation in all processes where heat is the chief factor. For
motive power either current may be used, only the electromotors have to
be constructed with reference to the kind of current that is used.
The rotary transformer, which may be driven by any power, consists of a
wheel carrying a rotating commutator so arranged with reference to
brushes that deliver the current to the commutator and carry it away
from the same, that the brushes leading out from the transformer will
always have impulses of the same polarity delivered to them. In the
parlance of the craft, the transformers that are used to change the
voltage from high to low, or vice versa, are called "static
transformers," simply because they are stationary, we suppose. The
others are called rotary, or moving transformers, to distinguish them
from the other forms. The operation of the latter is purely mechanical,
while the former is electrical. In some instances where the static
transformers are very large they develop a great amount of heat, so much
that it is necessary to devise means for dissipating it as fast as
created. In some instances this is done by air-currents forced through
them, but in others, where they are very large, oil is kept circulating
through the transformer from a tank that is elevated above it, the oil
being pumped back by a rotary pump into the tank where it is cooled by a
coil of pipe located in the oil, through which cold water is continually
circulating. By this means cold oil is constantly flowing down through
the transformer, where it absorbs the heat, which in turn is pumped back
into the tank, where it is cooled.
Having now traced the energy from the water-wheel through the various
transformations and having described in a very general way the apparatus
both for generating electricity and for transforming it to the right
voltage necessary for the various uses to which it is put, we will
proceed in our next chapter to follow it out to the points where it is
delivered, and trace it through its processes, and the part it plays in
creating the products of these various commercial establishments.