When servicing a Radio receiver it is sometimes necessary to wind a resistance (resistor) when replacement parts are not available. A well equipped service shop always carries in stock resistance wires. In making resistors the value wanted must be known in ohms, and then this value can be built into a resistor by carefully calculating the length of the wire, taking into consideration the size of wire used and the kind of material of which the wire is composed.
While copper is regularly considered a good conductor, it must not be forgotten that even the best conductors offer some resistance to current flow, and that the smaller the conductor, the greater the resistance.
In comparing different materials, some standard of unit dimensions must be adopted. The commercial copper wire standard generally used is the Annealed Copper Standard (See
Fig. 14—A micrometer.
Table No. 1) recommended by the United States Bureau of Standards of Washington, D.C. When a wire is referred to by a certain number, as for example, No. 16 or No. 32, this number means the gauge of the wire. It is a way of specifying the diameter.
A number of wire gauges differing slightly from each other, have been originated by different manufacturers of wire, but the one generally used in this country is the B& S Gauge (Brown & Sharpe Manufacturing Co.) commonly called the American Wire Gauge.
You will notice in Table No. 1 that the gauge (size) of copper wire ranges from No. 0000, which is one-half of an inch in diameter, to No. 40, which is as thin as a hair. In other words, the size decreases as the gauge numbers increase.
