Oscar F. Dame
Electricians speak of a coil of wire measuring 3 ohms, or an electric incandescent lamp measuring 660 ohms, and the term is such a common one that many amateurs use the word without really comprehending its exact meaning. Mr. Ohm was an electrical experimenter whose researches in electrical matters, especially in relation to the laws of current flow, caused his name to be given to the unit of electrical resistance.
When a current of electricity is passed through a conductor, the conductor offers a resistence of obstruction to its passage, and the amount of current that passes in every instance will depend on the magnitude of this resistance. The resistance of a conductor varies directly as the length of the conductor ; inversely as the area of cross section, and also upon the kind of substance composing the conductor. Simplified, this means that a piece of wire of a given gauge, seven in. long, will have seven times the resistance of a piece only one in. long. The larger the conductor the less the resistance, is illustrated by noting that a foot of No. 16 gauge wire has much less resistance than a foot of No. 36 gauge. We also learn that some metals are not such good conductors as others, and consequently a given gauge of copper wire and iron wire will have different resistances. All kinds of metals, therefore, have a factor of resistance used in calculations. When we know the length of a copper wire in feet and its diameter in mills, we can multiply the standard coefficient of copper, 10.18, by length in feet and divide this by the diameter in mills, squared, and get the exact resistance. For example: No. 36 copper wire is 5 mills in diameter, and to find the resistance of a piece 1000 ft. long, we have 10.18 times 1000 divided by 5 squared equals 407.2 ohms.
Ohms law explains that a unit of resistence depends upon certain relations existing between current and electro-motive force. Ohm found that when a current was passed through a wire or other conductor, the relation which existed between resistance current and electro-motive force could be expressed:Current, (Amperes) = E.M.F./Ohms
The correct difinition of an ohm, may therefore be as follows:
An ohm is that resistance through which a pressure of one volt will cause a current of one ampere to flow.
The catalogs of many manufacturers and electrical supply houses contain tables showing, among other things, the resistances of copper wires of all gauges by the pound and foot, also the diameter of these gauges in inches and mills and the number of feet in the pound.