ELECTRICAL MEASUREMENT.
Having given a short account of some
of the sources of electricity, let us now proceed
to describe some of the practical uses to which it
is put, and at the same time describe the operation
of the appliances used. Before proceeding further,
however, we ought to tell how electricity is measured.
We have pounds for weight, feet and inches for lineal
measure, and pints, quarts, gallons, pecks and bushels
for liquid and dry measure, and we also have ohms,
volts, amperes and ampere-hours for electricity.
When a current of electricity flows
through a conductor the conductor resists its flow
more or less according to the quality and size of the
conductor. Silver and copper are good conductors.
Silver is better than copper. Calling silver
100, copper will be only 73. If we have a mile
of silver wire and a mile of iron wire and want the
iron wire to carry as much electricity as the silver
and have the same battery for both, we will have to
make the iron wire over seven times as large.
That is, the area of a cross-section of the iron wire
must be over seven times that of the silver wire.
But if we want to keep both wires the same size and
still force the same amount of current through each
we must increase the pressure of the battery connected
with the iron wire. We measure this pressure
by a unit called the “volt,” named for
Volta, the inventor or discoverer of the voltaic battery.
The volt is the unit of pressure or electromotive
force. (In all these cases a “unit” is
a certain amount or quantity as of resistance,
electromotive force, etc. fixed upon
as a standard for measuring other amounts of the same
kind.)
The iron wire offers a resistance
that is about seven times greater than silver to the
passage of the current. To illustrate by water
pressure: If we should have two columns of water,
and a hole at the bottom of each column, one of them
seven times larger than the other, the water would
run out much faster from the larger hole if the columns
were the same height. Now, if we keep the column
with the larger hole at a fixed height a certain amount
of water will flow through per second. If we
raise the height of the column having the small hole
we shall reach a point after a time when there will
be as much water flow through the small hole per second
as there is flowing through the large hole. This
result has been accomplished by increasing the pressure.
So, we can accomplish a similar result in passing
electricity through an iron wire at the same rate
it flows through a silver wire of the same size, by
increasing the pressure, or electromotive power; and
this is called increasing the voltage.
The quality of the iron wire that
prevents the same amount of current from flowing through
it as the silver is called its resistance. The
unit of resistance, as mentioned in the last chapter,
is called the ohm, and the more ohms there are in
a wire as compared with another, the more volts we
have to put into the battery to get the same current.
The unit for measuring the current
is called the “ampere,” named after the
French electrician, A. M. Ampere (1789-1836).
Now, to make practical application
of these units. The volt is the potential or
pressure of one cell of battery called a standard cell,
made in a certain way. The electromotive force
of one cell of a Daniell battery is about one volt.
One ohm is the resistance offered to the passage of
a current having one volt pressure by a column of mercury
one millimeter in cross-section and 106.3 centimeters
in length. Ordinary iron telegraph-wire measures
about thirteen ohms to the mile. Now connect
our standard cell one volt through
one ohm resistance and we have a current of one ampere.
Unit electromotive force (volt) through unit resistance
(ohm) gives unit of current (ampere). It is not
the intention to treat the subject mathematically,
but I will give you a simple formula for finding the
amount of current if you know the resistance and the
voltage. The electromotive force divided by the
resistance gives the current. C = E/R or current
(amperes) equals electromotive force (volts) divided
by the resistance (ohms).
But still further: One ampere
of current having one volt pressure will develop one
watt of power, which is equal to 1/746 of a horse-power.
(The watt is named in honor of James Watt, the Scottish
inventor of the steam-engine 1786-1813).
In other words, 746 watts equal one horse-power.
By multiplying volts and amperes together we get watts.
If we want to carry only a small current
for a long distance we do not need to use large cells,
but many of them. We increase the pressure or
voltage by increasing the number of cells set up in
series. If we have a wire of given length and
resistance and find we need 100 volts to force the
right amount or strength of current through it, and
the electromotive force of the cells we are using
is one volt each, it will require 100 cells.
If we have a battery that has an E. M. F. of two volts
to the cell, as the storage-battery has, fifty cells
would answer. If we want a very strong current
of great volume, so to speak, for electric light or
power, and use a galvanic battery, we should have
to have cells of large surface and lower resistance
both inside and outside the cells.
When dynamos are used they are so
constructed that a given number of revolutions per
minute will give the right voltage. In fact, the
dynamo has to be built for the amount of current that
must be delivered through a given resistance.
The same holds good for a dynamo as for a galvanic
battery. If any one factor is fixed, we must adapt
the others to that one in order to get the result
we want. There are many other units, but to introduce
them here would only confuse the reader. The advanced
student is referred to the text-books.
With this much as a preliminary we
are prepared to take up the applications of electricity,
which to most people will be more interesting than
what has gone before.