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RESISTANCE 37
by various metals to the passage of electric current is directly proportional to the increase or decrease in temperature. (19) All resistance calculations are based on a normal temperature of 75 degrees Fahrenheit which equals 24 degrees centigrade.
(20) To calculate resistance of metals, what is known as the "temperature co-efficient" is used. (21) The tables of wire resistance are based upon the resistance offered by a mil foot (22) (one foot of wire of same composition as that being tested, 1/1000 of an inch in diameter) of the wire at normal temperature (75 degrees Fahr. ) . The co-efficient is the fraction of an ohm change in resistance offered for each degree of change in temperature.
To Ascertain Resistance
(23) To calculate the resistance of any wire its temperature co-efficient must be known. Multiply the coefficient by the number of degrees of its temperature away from normal, and add the result to or subtract it from the resistance at normal temperature, depending upon whether the temperature is higher or lower. Assuming the temperature co-efficient of a metal to be .001 and resistance at normal temperature 10 ohms per mil foot, what will be resistance at 175 degrees? We find the difference in temperature to be 175 — 75 = 100 degrees. Since resistance increases .001 per degree increase in temperature we have .001 X 100 = .1 of an ohm. Multiplying resistance at normal temperature, 10 ohms, by fractional increase .1, we have 10 X .1 = 1 ohm increase; hence if the total resistance at normal be 10 ohms, the resistance at 175 will be 11 ohms per mil foot.
Loss Through Resistance
(24) Resistance is power dissipated and lost. (25) Some resistance is unavoidable and high resistance is necessary under certain conditions but (26) it is an important function of the projectionist to eliminate all unnecessary resistance, conserving electric power by preventing its waste.