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36 RICHARDSON'S BLUEBOOK OF PROJECTION
wire is not exceeded, the heat is perceptible only to a sensitive thermometer. A wire of any given composition and diameter will convey a certain amperage at any voltage without excessive resistance, exactly the same as a water pipe of given diameter and roughness of walls will convey a certain definite number of gallons of water per minute without undue resistance. (14) The point where resistance in a wire begins to rise above normal marks the normal capacity of copper wires. (15) Beyond that point the resistance becomes excessive. Pressure used to overcome resistance appears in the form of heat.
If we attempt to force wires in excess of their rated capacity as shown by the underwriters' table of figures, they will generate heat, becoming red or even white hot, finally fusing and thus stopping the current flow, sometimes starting fires. Certainly the insulation will be quickly injured and perhaps entirely ruined. Overloading circuits is expensive and dangerous.
Exactly as in the case of water pipes, with any given current flow, resistance is decreased (a) as the wire diameter is increased, or (b) as the wTire length is decreased or (c) as its composition is changed to one of greater conductivity. Resistance is increased with (a) decrease in wire diameter (b) increase in length of wire (c) change of wire composition to one of less conductivity.
Resistance and Temperature
(16) The resistance of everything used for the conduction of electric current in projection work increases as its temperature is raised ; only carbon is excepted from this rule. With carbon the rule is reversed. Increase in temperature lowers its resistance. (17) The filament of a carbon filament incandescent lamp offers about twice the resistance when it is cold than at normal operating temperature. Incidentally it might be remarked in passing that the resistance of insulation materials and liquids is usually decreased as temperature increases.
(18) The increase or decrease of resistance presented