w
out of one end of the wires, and in at the other, the drum will
require more and more power to turn it, in proportion to the amount of
electricity we permit to flow. Thus, if one electric light is turned
on, the drum will press back with a certain strength on the water
wheel; if one hundred lights are turned on it will press back one
hundred times as much. Providing there is enough power in the water
wheel to continue turning the drum at its predetermined speed, the
dynamo will keep on giving more and more electricity if asked to,
until it finally destroys itself by fire. You cannot take more power,
in terms of electricity, out of a dynamo that you put into it, in
terms of mechanical motion. In fact, to insure flexibility and
constant speed at all loads, it is customary to provide twice as much
water wheel, or engine, power as the electrical rating of the dynamo.

[Illustration: An armature partly wound, showing slots and commutator]

We have seen that a water wheel is 85 per cent efficient under ideal
conditions. A dynamo's efficiency in translating mechanical motion
into electricity, varies with the type of machine and its size. The
largest machines attain as high as 90 per cent efficiency; the
smallest ones run as low as 40 per cent.

_Measuring Electric Power_

The amount of electricity any given dynamo can generate depends,
generally speaking, on two factors, i. e., (1) the power of the water
wheel, or other mechanical engine that turns the armature; and (2) the
size (carrying capacity) of the wires on this drum.

Strength, of electricity, is measured in _amperes_. An ampere of
electricity is the unit of the rate of flow and may be likened to a
gallon of water per minute.

In surveying for water-power, in Chapter III, we found that the
number of gallons or cubic feet of water alone did not determine the
amount of power. We found that the number of gallons or cubic feet
multiplied by the distance in feet it falls in a given time, was the
determining factor--pounds (quantity) multiplied by feet per
second--(velocity).

[Illustration: Showing the analogy of water to volts and amperes of
electricity]

The same is true in figuring the power of electricity. We multiply the
_amperes_ by the number of electric impulses that are created in the
wire in the course of one second. The unit of velocity, or pressure of
the electric current is called a _volt_. Voltage is the pressure which
causes electricity to flow