[CHAPTER XI]

NON-INDUCTIVE RESISTANCE DEVICES

It is often desired to introduce simple ohmic resistance into telephone circuits, in order to limit the current flow, or to create specific differences of potential at given points in the circuit.

Temperature Coefficient. The design or selection of resistance devices for various purposes frequently involves the consideration of the effect of temperature on the resistance of the conductor employed. The resistance of conductors is subject to change by changes in temperature. While nearly all metals show an increase, carbon shows a decrease in its resistance when heated.

The temperature coefficient of a conductor is a factor by which the resistance of the conductor at a given temperature must be multiplied in order to determine the change in resistance of that conductor brought about by a rise in temperature of one degree.

TABLE V

Temperature Coefficients

Pure MetalsTemperature Coefficients
CentigradeFahrenheit
Silver (annealed)0.004000.00222
Copper (annealed)0.004280.00242
Gold (99.9%)0.003770.00210
Aluminum (99%)0.004230.00235
Zinc0.004060.00226
Platinum (annealed)0.002470.00137
Iron0.006250.00347
Nickel0.00620.00345
Tin0.004400.00245
Lead0.004110.00228
Antimony0.003890.00216
Mercury0.000720.00044
Bismuth0.003540.00197

Positive and Negative Coefficients. Those conductors, in which a rise in temperature produces an increase in resistance, are said to have positive temperature coefficients, while those in which a rise in temperature produces a lowering of resistance are said to have negative temperature coefficients.