UNITS OF ELECTRICAL MEASUREMENT
Basic Electrical Theory
Electron current, or amperage, is described as the movement of free electrons through a
conductor. In electrical formulas, current is symbolized with a capital I, while in the laboratory
or on schematic diagrams, it is common to use a capital A to indicate amps or amperage (amps).
Now that we have discussed the concepts of voltage and current, we are ready to discuss a third
key concept called resistance. Resistance is defined as the opposition to current flow. The
amount of opposition to current flow produced by a material depends upon the amount of
available free electrons it contains and the types of obstacles the electrons encounter as they
attempt to move through the material. Resistance is measured in ohms and is represented by the
symbol (R) in equations. One ohm is defined as that amount of resistance that will limit the
current in a conductor to one ampere when the potential difference (voltage) applied to the
conductor is one volt. The shorthand notation for ohm is the Greek letter capital omega (W). If
a voltage is applied to a conductor, current flows. The amount of current flow depends upon the
resistance of the conductor. The lower the resistance, the higher the current flow for a given
amount of voltage. The higher the resistance, the lower the current flow.
In 1827, George Simon Ohm discovered that there was a definite relationship between voltage,
current, and resistance in an electrical circuit. Ohms Law defines this relationship and can be
stated in three ways.
Applied voltage equals circuit current times the circuit resistance. Equation (1-2) is a
mathematical respresentation of this concept.
E = I x R or E = IR
Current is equal to the applied voltage divided by the circuit resistance. Equation
(1-3) is a mathematical representation of this concept.