The Ampere
There are certain terms used in the electrical field to distinguish
various properties and qualities of the electrical current with which it
is well for the young experimenter to acquaint himself.
One of the first units usually required, in order to make intelligent
comparisons, is a unit of measure. The _quart_ is the unit of _measure_
commonly applied to liquids and is based upon the amount of space
occupied by a certain volume. The _pound_ is a unit of weight which
determines a certain amount of any substance by comparing the force
which gravity exerts in pulling it to the earth with the same effect of
gravity on another standard "weight."
Electric current is invisible and weightless, and for these and other
reasons cannot be measured by the quart or weighed by the pound. The
only way that it can be measured is by means of some of the effects
which it produces. Either the chemical, electro-magnetic, or the heating
effects may be made the basis of a system of measurement.
The first method used to measure electric current was the chemical one.
If a current is passed through a solution of a chemical called copper
_sulphate_ (blue vitriol) by means of two copper plates, _copper_ will
be deposited on one plate and dissolved from the other. If the current
is furnished by a battery the copper will be deposited on the plate
connected with the zinc of the battery. If the current is allowed to
flow for a short time and the two copper plates are then taken out and
weighed it will be found that one plate is considerably heavier than the
other.
The copper has been taken from one plate and deposited on the other by
the _electric currents_. The amount of electric current which will
deposit 1.177 grammes of copper in an hour is called an _ampere_. The
ampere is the unit of electrical current measurement, and implies
quantity or amount.
The chemical method of measuring current was at one time put to
practical service in the distribution of electric current for lighting
and power. Many years ago the house meters, used to measure the current,
consisted of a jar containing two copper plates. The current used in the
house would cause copper to deposit on one plate, and by weighing the
plate the power company could determine the amount of current used, and
thereby the amount of the bill. The meters nowadays make use of the
magnetic effects of the current instead of the chemical, as described
later on.
The Volt
For purposes of explanation the electric current may be likened to a
stream of water flowing through a pipe.
If you hold your thumb over the end of a water-pipe through which water
is flowing it will push your thumb away because of the _pressure_ which
the water exerts.
Electric currents also exert a _pressure_, only it is not called
pressure in electrical parlance, but, spoken of as _electromotive force_
or _potential_.
The pressure of the water enables it to pass through small openings and
to overcome the resistance offered by the pipe.
Wires and other electrical conductors do not offer a perfectly free path
to an electric current, but also possess a resistance. It is the
potential of the electro-motive force which overcomes the resistance and
pushes the current through the wire.
Advantage has been taken of the fact to fix a unit of electrical
pressure called the _volt_. The pressure of the water in a water-pipe is
measured in pounds, but the pressure of an electric current in a wire is
measured by _volts_. The volt is the unit of electrical force which will
cause a current of one ampere to flow through a resistance of one _ohm_.
The Ohm
The ohm is the unit of electrical resistance. The standard ohm is the
resistance offered by a column of pure mercury having a section of one
square millimeter and a length of 106.28 centimeters at a temperature of
0° centigrade.
The pressure which will force sufficient current through such a column
of mercury to deposit 1.177 grammes of copper in one hour is a volt, and
in doing so has passed a current of one ampere through a resistance of
one ohm.
The units ohm, ampere, and volt, were named in honor of the three great
electricians: Ohm, Ampère, and Volta.
These three units bear a very close relation to each other which is
explained by Ohm’s Law.
Ohm’s Law is a simple statement of facts which it is well for the young
electrician thoroughly to understand, for it might almost be said to be
the basis of design of almost all electrical instruments.
It is simply this: The strength of a current equals the voltage divided
by the resistance. It may be expressed in symbols by: _C = E/R_. Where C
is the current in amperes, E is the potential in volts, and R the
resistance in ohms.
By way of a simple example, we will suppose that a small telegraph
sounder is connected to a battery and that the voltage of the battery is
_ten volts_. We will further suppose that the resistance of the sounder
connecting wires and the battery itself is _five ohms_. Knowing these
two facts, it is very easy to find out how many amperes are flowing
through the sounder by substituting these values in the equation as
follows:
C = E/R
E = 10 volts and R = 5 ohms
therefore C = 10/5 or 2 amperes
In order to indicate fractions or very large values of the ampere, volt,
and ohm, it is customary to use the following terms:
Milli-volt = 1/1000 of a volt
Mill-ampere = 1/1000 of an ampere
Kilo-volt = 1000 volts
Meg-ohm = 1,000,000 ohms
The Watt
It is no doubt perfectly plain that the water in a certain size of pipe
at a pressure of 100 lbs. is more powerful than a stream of water in the
same size of pipe at 25 lbs. pressure.
Likewise a current of electricity represents more power at 100 volts
potential than the same current would at 25 volts. The unit of
electrical power is called the _watt_. A watt is represented by a
current of one ampere flowing through a wire at a potential of one volt.
The number of watts is found by multiplying the voltage by the amperage.
In the case of the sounder and battery used as an example to explain
Ohm’s Law, and where the voltage was 10 and the amperage found to be 2,
the number of watts is 10 x 2, or 20 watts.
Seven hundred and forty-six watts represent one electrical horse-power.
One thousand watts are called a _kilo-watt_.
The Coulomb
So far, none of the units have taken into consideration the element of
time.
If water should be permitted to run out of a pipe into a tank until ten
gallons had passed it would not be possible to tell at what rate the
water was flowing by knowing that ten gallons had passed unless it were
also known how long the water had been flowing. Ten gallons per minute
or ten gallons per hour would indicate the rate of flow.
One ampere flowing for one second is the electrical unit of flow. This
unit is called the _coulomb_.
One ampere flowing for one hour is called an _ampere hour_. The number
of ampere hours is found by multiplying the current in amperes by the
time in hours.
A battery may be said to have a capacity of 10 ampere hours. This means
that it will deliver one ampere for 10 hours (1 ampere x 10 hours = 10
ampere hours) or 2 amperes for 5 hours (2 amperes x 5 hours = 10 ampere
hours).
The same element of time enters into consideration in connection with
the watt. One watt flowing for one hour is a _watt hour_ and one
kilowatt flowing for one hour is a _kilo-watt hour_.
The Difference between Alternating and Direct Currents
There are two distinct kinds of electric current supplied for lighting
and power, one known as _direct_ current and the other as _alternating_.
A _direct current_ is one which passes in one direction only. It may be
represented by a straight line, as _A_ in Figure 88.
An alternating current is one which reverses its direction and passes
first one way and then the other. It may be represented by a curved
line, shown in Figure 88. It starts at _zero_, and gradually grows
stronger and stronger. Then it commences to die away until no current is
flowing. At this point it reverses and commences to flow in the opposite
direction, rising gradually and then dying away again.
This is repeated a definite number of times per second; when the current
rises from zero, reverses and returns to zero, it is said to pass
through a _cycle_.
[Illustration: Fig. 88.—Graphic Representation of a Direct and an
Alternating Current.]
The part of the curved line from _a_ to _b_ in Figure 88 represents the
first part of the current, when it is rising. From _b_ to _c_ represents
its fall. The point at which the curved line crosses the straight line
is zero. At _c_ the current crosses the line and commences to flow in
the opposite direction until it reaches _d_, at which point it dies away
and again crosses the line to flow in its original direction and _repeat
the cycle_.
In electrical parlance, that part of the current from _a_ to _c_ or from
_c_ to _e_ is known as an _alternation_. From _a_ to _e_ is called a
cycle.
The reason why alternating current is often used in place of direct
current is that it can be sent over the wires for long distances more
economically than direct current. This is more fully explained farther
on in the chapter dealing with a step-down transformer.
The number of _cycles_ taking place in one second is known as the
_frequency_ of the current. The usual _frequency_ of commercial
alternating currents is 60 cycles per second or 7200 alternations per
minute.