. A volt may be likened to the velocity in
feet per second of water in falling past a certain point. If you
think a moment you will see that this has nothing to do with quantity.
A pin-hole stream of water under 40 pounds pressure has the same
velocity as water coming from a nozzle as big as a barrel, under the
same pressure. So with electricity under the pressure of one volt or
one hundred volts.

One volt is said to consist of a succession of impulses caused by _one
wire cutting 100,000,000 lines of magnetic force in one second_. Thus,
if the strength of a magnet consisted of one line of force, to create
the pressure of one volt we would have to "cut" that line of force
100,000,000 times a second, with one wire; or 100,000 times a second
with one thousand wires. Or, if a magnet could be made with
100,000,000 lines of force, a single wire cutting those lines once in
a second would create one volt pressure. In actual practice, field
magnets of dynamos are worked at densities up to and over 100,000
lines of force to the square inch, and armatures contain several
hundred conductors to "cut" these magnetic lines. The voltage then
depends on the speed at which the armature is driven. In machines for
isolated plants, it will be found that the speed varies from 400
revolutions per minute, to 1,800, according to the design of dynamo
used.

[Illustration: Pressure determines volume of flow in a given time]

Multiplying amperes (strength) by volts (pressure), gives us _watts_
(power). Seven hundred and forty-six watts of electrical energy is
equal to one horsepower of mechanical energy--will do the same work.
Thus an electric current under a pressure of 100 volts, and a density
of 7.46 amperes, is one horsepower; as is 74.6 amperes, at 10 volts
pressure; or 746 amperes at one volt pressure. For convenience (as a
watt is a small quantity) electricity is measured in _kilowatts_, or
1,000 watts. Since 746 watts is one horsepower, 1,000 watts or one
kilowatt is 1.34 horsepower. The work of such a current for one hour
is called a _kilowatt-hour_, and in our cities, where electricity is
generated from steam, the retail price of a kilowatt-hour varies from
10 to 15 cents.

Now as to how electricity may be controlled, so that a dynamo will not
burn itself up when it begins to generate.

Again we come back to the analogy of water. The amount of water that
passes through a pipe in any given time, depends on the size of the
pipe, if