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HEATING EFFECTS OF CURRENTS.
Insurance rules for carrying capacity of wires.

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HEATING EFFECTS OF CURRENTS.-(Cont.)

Carrying capacity of insulated wires in mouldings.

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HEATING EFFECTS OF CURRENTS.—(Cont.) Bare copper in still air.

10°

Rise in temperature, degrees Centigrade.

20°

40°

80°

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THE

SPANS.

HE formulæ used in calculating these tables of lengths and strains in spans of wire are those of a catenary of small deflection. They are given in Weisbach's Mechanics of Engineering, page 297. (seventh American edition, translated by Eckley B. Coxe, A. M.)

In these tables the horizontal strain at the center of the span is given. The strain at any other point equals the strain at the center plus the weight of a length of the wire equal to the perpendicular distance of that point from the lowest point of the wire in the span. For ordinary spans this is negligible. For any given wire the longest possible span is one where the deflection is about one-third of the span.

The effects of temperature on the strains of wires in spans is at first sight so great as to render the other considerations of little importance. The table, page 53, is calculated on the assumption that the supports of the spans are perfectly rigid under all conditions of strain and that the wire is inelastic. This is never true in practice. The changes in direction in a pole line afford a chance for the strains, due to a shortening of the wire by a fall in temperature, to be taken up by a bending of the supports.

If the elastic limit of hard-drawn copper wire of 60 000 pounds breaking strain be taken at 20 000 pounds, then S will equal 20 000 divided by 3.85, the weight of a piece of copper one foot long and one square inch in section. This makes S equal 5 195. Looking at the table of values of S, page 50, this value for a span of 130 feet comes between a deflection of .003 and .004. In the same way the allowable deflection for any other span of hard-drawn copper could be found or for any other material by substituting the proper terms for the elastic limit and the weight per foot given above.

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