315
OF ENGINEERING FORMULÆ.
Telegraph Construction, by R. S. Brough—
continued.
Having found the position of the lowest point
of the curve, all the other particulars of the curve
can be found by the preceding formulae.
§ n. Stbains produced by Wire.
Straight Line.—The whole vertical pressure on
the supports of any line is obviously equal to the
whole weight of the wire on the line. When the
points of support are on the same level, and the
spans are equal, the vertical pressure on any one
support is equal to the weight of wire in one span.
In erecting an “ un-checked ” wire, we have practi-
cally to deal with the case of a chord passing over
smooth pulleys, and hence the strains along the
wire on opposite sides of the support are always
equal. The resultant horizontal strain, if any, ia
therefore P = (cos. i - cos.»') i,
where i and »' are the angles made by the wire
with the horizon on opposite sides of the support.
If, however, the supports be on the same level, and
the spans be equal, then i = i', and the resultant
horizontal strain is nil.
When the supports are not on the same level,
the distance to set them apart in order to make
« = i' can be calculated.
Angles.—Let 0 — the angle contained by the
wire.
= the supplement of the same.
R = the resultant strain due to
the wire.
Then
R = 2 t cos.~ = 21 sin. ~ = t/Jl (1 — cob. q>).