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.