the pressure is maintained uniform. In other words the
_resistance_ of the pipe to the flow of water determines the amount.
If the pipe be the size of a pin-hole, a very small amount of water
will escape. If the pipe is as big around as a barrel, a large amount
will force its way through. So with electricity. Resistance,
introduced in the electric circuit, controls the amount of current
that flows. A wire as fine as a hair will permit only a small quantity
to pass, under a given pressure. A wire as big as one's thumb will
permit a correspondingly greater quantity to pass, the pressure
remaining the same. The unit of electrical resistance is called the
_ohm_--named after a man, as are all electrical units.

_Ohm's Law_

The _ohm_ is that amount of _resistance_ that will permit the passage
of _one ampere_, under the pressure of _one volt_. It would take two
volts to force two amperes through one ohm; or 100 volts to force 100
amperes through the resistance of one ohm. From this we have Ohm's
Law, a simple formula which is the beginning and end of all electric
computations the farmer will have to make in installing his
water-power electric plant. Ohm's Law tells us that the density of
current (amperes) that can pass through a given resistance in ohms (a
wire, a lamp, or an electric stove) equals _volts_ divided by
_ohms_--or _pressure_ divided by _resistance_. This formula may be
written in three ways, thus:

C = E/R, or R = E/C or, E = C x R. Or to express the same thing in
words, _current_ equals _volts_ divided by _ohms_; _ohms_ equals
_volts_ divided by _current_; or _volts_ equals _current_ multiplied
by _ohms_. So, with any two of these three determining factors known,
we can find the third. As we have said, this simple law is the
beginning and end of ordinary calculations as to electric current, and
it should be thoroughly understood by any farmer who essays to be his
own electrical engineer. Once understood and applied, the problem of
the control of the electric current becomes simple a b c.

_Examples of Ohm's Law_

Let us illustrate its application by an example. The water wheel is
started and is spinning the dynamo at its rated speed, say 1,500
r.p.m. Two heavy wires, leading from brushes which collect electricity
from the revolving armature, are led, by suitable insulated supports
to the switchboard, and fastened there. They do not touch each other.
Dynamo mains must not be permitted to touch each ot