335
OF ENGINEERING FORMULÆ.
Electrical Formula, by Paget Higgs, LL.D., C.E.,
and R. S. Brough—continued.
- 1-079 volts; of Bunsen’s = 1-88 volts; of
Marie-Davy’s = 1'524 volts; and of LeclancLé’s
= 1’48 volts.
18. The resistance of one Siemens’ mercury
unit = 0-9536 ohm. (B. A. Report.) The re-
sistance of one ohm = 1 • 0493 S. U.
19. Copper weighs about 559 lbs. per cubic foot.
Sp. gr. 8 • 95.
A prism of pure copper one metre long and one
square millimetre in section weighs about 8’95
grammes; and its resistance is about 0-01642
(Jim at 0° C., and increases about 0 • 388 per cent,
per 1° C. (Maxwell.)
The resistance of a prism of pure copper one
metre long and weighing one gramme is about
0-14696 ohm at 0° C. The weight per knot of a
telegraphic pure copper wire is 18430 d2, d being
taken in decimals of an inch. The diameter of a
pure copper wire, weighing w lbs. per knot, is
7-366 „/w mils. The resistance per knot of a pure
1192*45
copper wire, weighing w lbs., is —-— ohms, or
~ Siemens’ units at 75° F.
w
20. Guttapercha weighs about 61 lbs. per cubic
foot, __
The resistance of a cubic knot of guttapercha
is 2100 megohms at 75° F., and its capacity about
0'0687 microfarad, The weight of guttapercha