<h2><SPAN name="CHAPTER_IX" id="CHAPTER_IX"></SPAN>CHAPTER IX.</h2>
<h3>ELECTRICAL MEASUREMENT.</h3>
<p>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, ampères and ampère-hours for
electricity.</p>
<p>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<span class="pagenum"><SPAN name="Page_84" id="Page_84"></SPAN></span>
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.)</p>
<p>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<span class="pagenum"><SPAN name="Page_85" id="Page_85"></SPAN></span>
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.</p>
<p>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.</p>
<p>The unit for measuring the current is called
the "ampère," named after the French electrician,
A. M. Ampère (1789-1836).</p>
<p>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 ampère.<span class="pagenum"><SPAN name="Page_86" id="Page_86"></SPAN></span>
Unit electromotive force (volt) through unit
resistance (ohm) gives unit of current (ampère).
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. <i>C</i> = <i>E</i> / <i>R</i> or current
(ampères) equals electromotive force (volts)
divided by the resistance (ohms).</p>
<p>But still further: One ampère 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 ampères together we get watts.</p>
<p>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<span class="pagenum"><SPAN name="Page_87" id="Page_87"></SPAN></span>
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.</p>
<p>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.</p>
<p>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.</p>
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