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This is defined1 as "a device, the primary purpose of which is to introduce resistance into an electric circuit," Unfortunately, a resistor contains small amounts of "residual" inductance and capacitance. These unwanted properties become of extreme importance at radio frequencies. An equivalent circuit of a resistor is shown in Fig. 2.

equivalent circuit of a resistor
Figure 2. An equivalent circuit of a resistor. The inductance and capacitance are undesired.

Wire-Wound Resistors. Such resistors are used extensively in communication circuits, particularly where the frequency is low, where the current is relatively high, and where the resistors must be very stable and of the order of a few hundred thousand ohms and less. The methods of winding resistors to minimize inductance are such that the magnetic field produced is kept to the minimum by winding the resistance wire on a thin card, or by arranging the wires so that the flux-producing tendency of one wire is canceled by that of an adjacent wire.3 Unwanted capacitances are minimized by reducing metallic areas and separating metallic portions between which a potential difference will exist.

Composition Resistors. This general heading includes several types of resistors. One is the so-called carbon resistor composed of a carbon composition formed into a short straight rod that is often encased in insulation. Another is the so-called metallized type consisting of a thin conducting film on an insulating rod, such as glass, and encased in insulation.

Composition resistors are particularly suited for radio because they are readily made with very high resistances, and because the residual capacitance and inductance are very low.

Last Update: 2011-05-14