Voltage is a measure of electrical pressure. In many respects, it is analogous to hydraulic pressure. The unit of voltage is the volt, abbreviated V. Thus we speak of a 12-V or 24-V system. Prefixes, keyed to the decimal system, expand the terms so we do not need to contend with a series of zeros. The prefix K stands for kilo, or 1000. A 50-kV powerline delivers 50,000V. At the other end of the scale, milli means 1/1000, and one millivolt (1 mV) is a thousandth of a volt.
The ampere, shortened sometimes to amp and abbreviated A is a measure of the quantity of electrons flowing past a given point in the circuit per second. One ampere represents the flow of 6.25 X 1018 electrons per second. Amperage is also referred to as current intensity or quantity. From the point of view of the loads on the circuit, the amperage is the draw. A free-running starter motor might draw 100A, and three times as much under cranking loads.
Resistance is measured in units named after G. S. Ohm. Ohms are expressed by the last letter of the Greek alphabet, omega (!1). Thus we might speak of a 200!1 resistance.
The resistance of a circuit determines the amount of current that flows for a given applied voltage. The resistance depends on the atomic structure of the conductor—how tightly the electrons are held captive in their orbits—and on certain physical characteristics. The broader the cross-sectional area of the conductor, the lesser opposition to the current. And the longer the path formed by the circuit between the poles of the voltage source, the more is the resistance. Think of these two dimensions in terms of ordinary plumbing. The resistance to the flow of a liquid in a pipe is inversely related to its diameter (decreases as diameter increases) and directly related to its length. Resistance in the pipe produces heat, exactly as does resistance in an electrical conductor.
Resistance is generally thought of as the electrical equivalent of friction—a kind of excise tax that we must pay to have electron movement. There are, however, pos- itive uses of resistance. Resistive elements can be deliberately introduced in the cir- cuit to reduce current in order to protect delicate components. The heating effect of resistance is used in soldering guns and irons and in the glow plugs employed as starting aids in diesel engines.