The Volt,
The Ampère,
The Ohm.
We will explain these in their order.
[Illustration: Fig. 1]
_The Volt._--This term may be better understood by making a comparison
with something you all know of. Suppose we have a tank containing
one hundred gallons of water, and we want to discharge it through a
half-inch pipe at the bottom of the tank. Suppose, further, that we
wanted to make the water spout upward, and for this purpose the pipe
was bent upward as in Fig. 1.
If you opened the tap the water would spout out and upward as in Fig. 1.
[Illustration: Fig. 2]
The cause of its spouting upward would be the _weight_ or _pressure_ of
the water in the tank. This pressure is reckoned as so many _pounds_ to
the square inch of water.
Now, if the tank were placed on the roof of the house and the pipe
brought to the ground as shown in Fig. 2, the water would spout up very
much higher, because there would be _many more pounds_ of pressure on
account of the height of the pipe.
So, you see, the force or pressure of water is measured in pounds, and,
therefore, a pound is the unit of pressure, or force, of water. Now, in
electricity the unit of pressure, or force, is called a volt.
This word "volt" does not mean any weight, as the word "pound"
weight does. You all know that if you have a pound of water you must
have something to hold it, because it has weight, and, consequently,
occupies some space. But _electricity itself has no weight_ and
therefore cannot occupy any space.
When we desire to carry water into a house or other building we do so
by means of hollow pipes, which are usually made of iron. This is the
way that water is brought into houses in cities and towns, so that it
may be drawn and used in any part of a dwelling. Now, the principal
supply usually comes from a reservoir which is placed up on high ground
so as to give the necessary pounds of pressure to force the water up to
the upper part of the houses. If some arrangement of this kind were not
made we could get no water in our bedrooms, because, as you know, water
will not rise above its own level unless by force.
The water cannot escape as long as there are no holes or leaks in the
iron pipes, but if there should be the slightest crevice in them the
water will run out.
In electricity we find similar effects.
The electricity is carried into houses by means of wires which are
covered, or _insulated_, with various substances, such, for instance,
as rubber. Just as the iron of the pipes prevents the water from
escaping, the insulation of the wire prevents the escape of the
electricity.
Now, if we were to cause the pounds of pressure of water, in pipes of
ordinary thickness, to be very greatly increased, the pipes could not
stand the strain and would burst and the water escape. So it is with
electricity. If there were too many volts of pressure the insulation
would not be sufficient to hold it and the electricity would escape
through the covering, or insulation, of the wire.
It is a simple and easy matter to stop the flow of water from an
ordinary faucet by placing your finger over the opening. As the water
cannot then flow, your finger is what we will call a non-conductor and
the water will be retained in the pipe.
We have just the same effects in electricity. If we place some
substance which is practically a non-conductor, or insulator, such as
rubber, around an electric wire, or in the path of an electric current,
the electricity, acted upon by the volts of pressure, cannot escape,
because the insulation keeps it from doing so, just as the iron of the
pipe keeps the water from escaping. Thus, you see, the volt does not
itself represent electricity, but only the pressure which forces it
through the wire.
There are other words and expressions in electricity which are
sometimes used in connection with the word "volt." These words are
"pressure" and "intensity." We might say, for instance, that a certain
dynamo machine had an electromotive force of 110 volts; or that the
intensity of a cell of a battery was 2 volts, etc.
We might mention, as another analogy, the pressure of steam in a
boiler, which is measured or calculated in pounds, just as the pressure
of water is measured. So, we might say that 100 pounds steam pressure
used through the medium of a steam-engine to drive a dynamo could thus
be changed to electricity at 100 volts pressure.
_The Ampère._--Now, in comparing the pounds pressure of water with the
volts of pressure of electricity we used as an illustration a tank of
water containing 100 gallons, and we saw that this water had a downward
force or pressure in pounds. Let us now see what this pressure was
acting upon.
It was forcing the quantity of water to spout upward through the end of
the pipe. Now, as the quantity of water was 100 gallons, it could not
all be forced at once out of the end of the pipe. The pounds pressure
of water acting on the 100 gallons would force it out at a _certain
rate_, which, let us say, would be one gallon per minute.
This would be the _rate of the flow_ of water out of the tank.
Thus, you see, we find a second measurement to be considered in
discharging the water-tank. The first was the force, or pounds of
pressure, and the second the _rate_ at which the quantity of water was
being discharged per minute by that pressure.
This second measurement teaches us that a _certain quantity_ will pass
out of the pipe in a _certain time_ if the pressure is steady, such
quantity depending, of course, on the size or friction resistance of
the pipe.
In electricity the volts of pressure act so as to force the quantity
of current to _flow through the wires at a certain rate_ per second,
and the rate at which it flows is measured in ampères. For instance,
let us suppose that an electric lamp required a pressure of 100 volts
and a current of one ampère to light it up, we should have to supply a
current of electricity flowing at the rate of one ampère, acted upon by
an electromotive force of 100 volts.
You will see, therefore, that while the volt does not represent any
electricity, but only its pressure, the ampère represents the _rate of
flow_ of the current itself.
You should remember that there are several words sometimes used in
connection with the word "ampère"--for instance, we might say that a
lamp required a "current" of one ampère or that a dynamo would give a
"quantity" of 20 ampères.
_The Ohm._--You have learned that the _pressure_ would discharge the
_quantity_ of water at a certain rate through the pipe. Now, suppose we
were to fix _two_ discharge-pipes to the tank, the water would run away
very much quicker, would it not? If we try to find a reason for this,
we shall see that a pipe can only, at a given pressure, admit so much
water through it at a time.
Therefore, you see, this pipe would present a certain amount of
_resistance_ to the passage of the total quantity of water, and would
only allow a limited quantity at once to go through. But, if we were to
attach two or more pipes to the tank, or one large pipe, we should make
it easier for the water to flow, and, therefore, the total amount of
resistance to the passage of the water would be very much less, and the
tank would quickly be emptied.
Now, as you already know, water has substance and weight and therefore
occupies some space, but electricity has neither substance nor
weight, and therefore cannot occupy any space; consequently, to carry
electricity from one place to another we do not need to use a pipe,
which is hollow, but we use a solid wire.
These solid wires have a certain amount of _resistance_ to the passage
of the electricity, just as the water-pipe has to the water, and (as
it is in the case of the water) the effect of the resistance to the
passage of electricity is greater if you pass a larger quantity through
than a smaller quantity.
If you wanted to carry a quantity of electricity to a certain distance,
and for that purpose used a wire, there would be a certain amount of
resistance in that wire to the passage of the current through it; but
if you used two or more wires of the same size, or one large wire, the
resistance would be very much less and the current would flow more
easily.
Suppose that, instead of emptying the water-tank from the roof through
the pipe, we had just turned the tank over and let the water all pour
out at once down to the ground. That would dispose of the water very
quickly and by a short way, would it not? That is very easy to be
seen, because there would be _no resistance_ to its passage to the
ground.
Well, suppose we had an electric battery giving a certain quantity of
current, say five ampères, and we should take a large wire that would
offer no resistance to that quantity and put it from one side of the
battery to the other, a large current would flow at once and tend to
exhaust the battery. This is called a _short circuit_ because there is
little or no resistance, and it provides the current with an easy path
to escape. Remember this, that _electricity always takes the easiest
path_. It will take as many paths as are offered, but the largest
quantity will always take the easiest.
As the subject of resistance is one of the most important in
electricity, we will give you one more example, because if you can
obtain a good understanding of this principle it will help you to
comprehend the whole subject more easily in your future studies.
We started by comparison with a tank holding 100 gallons of water,
discharging through a half-inch pipe, and showed you that the pounds
of pressure would force the quantity of gallons through the pipe. When
the tap was first opened the water would spout up very high, but as
the water in the tank became lower the pressure would be less, and,
consequently, the water would not spout so high.
So, if it were desired to keep the water spouting up to the height
it started with, we should have to keep the tank full, so as to have
the same pounds of pressure all the time. But, if we wanted the water
to spout still higher we should have to use other means, such as a
force-pump, to obtain a greater pressure.
Now, if we should use too many pounds pressure it would force the
quantity of water more rapidly through the pipe and would cause the
water to become heated because of the resistance of the pipe to the
passage of that quantity acted upon by so great a pressure.
This is just the same in electricity, except that the wire itself would
become heated, some of the electricity being turned into heat and lost.
If a wire were too small for the volts pressure and ampères of current
of electricity the resistance of such wire would be overcome, and it
would become red-hot and perhaps melt. Electricians are therefore
very careful to calculate the resistance of the wires they use before
putting them up, especially when they are for electric lighting, in
order to make allowances for the ampères of current to flow through
them, so that but little of the electricity will be turned into heat
and thus rendered useless for their purpose.
The unit of resistance is called the _ohm_ (pronounced like "home"
without the "h").
All wires have a certain resistance per foot, according to the nature
of the metal used and the size of the wire--that is to say, the finer
the wire the greater number of ohms resistance it has to the foot.
Water and electricity flow under very similar conditions--that is
to say, each of them must have a channel, or conductor, and each of
them requires pressure to force it onward. Water, however, being a
tangible substance, requires a hollow conductor; while electricity,
being intangible, will flow through a solid conductor. The iron of
the water-pipe and the insulation of the electric wire serve the same
purpose--namely, that of serving to prevent escape by reason of the
pressure exerted.
There is another term which should be mentioned in connection with
resistance, as they are closely related, and that is _opposition_.
There is no general electrical term of this name, but, as it will be
most easily understood from the meaning of the word itself, we have
used it.
Let us give an example of what opposition would mean if applied to
water. Probably every one knows that a water-wheel is a wheel having
large blades, or "paddles," around its circumference.
When the water, in trying to force its passage, rushes against one of
these paddles it meets with its opposition, but overcomes it by pushing
the paddle away. This brings around more opposition in the shape of
another paddle, which the water also pushes away. And so this goes on,
the water overcoming this opposition and turning the wheel around, by
which means we can get water to do useful work for us.
You must remember, however, that it is only by putting opposition in
the path of a pressure and quantity of water that we can get this work.
The same principle holds good in electricity. We make electricity in
different ways, and in order to obtain useful work we put in its path
the instruments, lamps, or machines which offer the proper amount of
resistance, or opposition, to its passage, and thus obtain from this
wonderful agent the work we desire to have done.
You have learned that three important measurements in electricity are
as follows:
The _volt_ is the practical unit of measurement of _pressure_;
The _ampère_ is the practical unit of measurement of the _rate of
flow_; and
The _ohm_ is the practical unit of measurement of _resistance_.