336 Molesworth’s pocket-book
Electrical Formulae, by Paget Higgs, LL.D., C.E.,
and R. S. Brough—continued.
, D2 — d2
per knot in any core is approximately -
lbs., where D is outer and d inner diameter of
g.p. sheath in mils. The diameter of a gutta-
percha core with solid conductor weighing w lbs.
per knot, and g.p. weighing W lbs. per knot, is
V54'3 w + 486 W mils.
Let w' = resistance of guttapercha at t'° F.,
w" — resistance of guttapercha at t"° F.,
then log. w" = log. w' — (t” — t'~) log. 0’0399.
21. Hooper’s material weighs about 73| lbs.
per cubic foot.
The resistance of a cubic knot of Hooper’s
material is 40950 megohms at 75° F., and its
capacity about 0’0543 microfarad.
22. Iron weighs about 481 lbs. per cubic foot.
Sp.gr. 7-7. . .
The resistance of a prism of soft iron one metre
long and one square millimetre in section is about
0-096 olnn at 0° C., and increases about 0’7 per
cent, per 1° G.
The resistance of a prism of soft iron one metre
long and weighing one gramme is about 0‘7392
ohm at 0° O.
The resistance of 100 yards No. 8 iron wire is
roughly one ohm.
23. German silver weighs about 530 lbs. per
cubic foot. Sp. gr. 8’5.
A prism of German silver one metre long and