ELECTRICAL UNITS 9 5
per in one hour is a volt, and in doing so has passed a cur-
rent of one ampere through a resistance of one ohm.
The units ohm, ampere, and volt, were named in honor
.of the three great electricians: th, Ampere, 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 = g. 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
E = 10 volts and R = 5 ohms
therefore C = 139 or 2 amperes
In order to indicate fractions or very large values of the am-
pere, volt, and ohm, it is customary to use the following terms: