Advantage has been taken of the fact to fix a unit of electrical pressure called the volt. The pressure of the water in a water-pipe is measured in pounds, but the pressure of an electric current in a wire is measured by volts. The volt is the unit of electrical force which will cause a current of one ampere to flow through a resistance of one ohm.
The Ohm
The ohm is the unit of electrical resistance. The standard ohm is the resistance offered by a column of pure mercury having a section of one square millimeter and a length of 106.28 centimeters at a temperature of 0° centigrade.
The pressure which will force sufficient current through such a column of mercury to deposit 1.177 grammes of copper in one hour is a volt, and in doing so has passed a current of one ampere through a resistance of one ohm.
The units ohm, ampere, and volt, were named in honor of the three great electricians: Ohm, Ampère, 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 = E/R. 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/R