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42 MOTION PICTURE PROJECTION
less is its resistance. Therefore, if two wires have the same length, but one has a cross-section three times that of the other, the resistance of the former is one-third that of the latter.
As the area of a circle is proportional to the square of its diameter, it follows that the resistances of round conductors are inversely proportional to the squares of their diameters.
Specific Resistance. — The specific resistance of a substance is the resistance of a portion of that substance of unit length and unit cross-section at a standard temperature. The units commonly used are the centimeter or the inch, and the temperature that of melting ice. The specific resistance may therefore be said to be the resistance (usually stated in microhms) of a centimeter cube or of an inch cube at the temperature of melting ice. If the specific resistances of two substances are known, then their related assistance is given by the ratio of the specific resistance.
Calculation of Resistance. — It is evident that resistance varies directly as the length, inversely as the cross-sectional area, and depends upon the specific resistance of the material.
If a circuit is made up of several different materials joined in series with each other, the resistance of the circuit is equal to the sum of the resistances of its several parts. In calculating the resistance of such a circuit, the resistance of each part should first be calculated, and the sum of these resistances will be the total resistance of the circuit.
Resistance Affected by Heating. — The resistance of metals depends upon the temperature, and the resistance is increased by heating. The