3C3 OF ENGINEERING FORMULM
Relative Speed op Working Cables of similar Length.
(Sir W. Thompson.)
D — Diameter of insulator.
„ conductor.
S
= Relative speed = 200 e
2
log-e
p
d '
1 «I®- D d 8 ei r. b d s 1 D I D d 3
o-i 10 •1252 0-5 2- •9421 0*8 1-25 •HS4
0'2 * •3500 0-6 i-es '9996 1-11 •4684
0’3 3-33 •5«91 0'6065 1’649 1'0000
0*4 2-5J •7971 0-7 1-429 •9551
Actual Speed of WoRKifc in Cables.
L — Length in kuots. S == Speed woeds per minute.
Red Sea, L =b 629, S = 11; Alexandria, Malta, L = 92»,
S = 19; Persian Gulf, L = S =» Si; Atlantic (1865),
h zs 1896, S 17; Atlantic (IrftiB), I, =3 1857, Ö =: 17; French
Atlantic, L =5 2584»S = 15.
Lightning. Conductobs.
Necessary conditions.—1. Good continuous conduction.
2. Ample earth contact, capable of being frequently examined.
3. Termination in points not subject to corrosion. 4. No por«
tion of the building outside the limits of the conductor to be
without a point eounected to the conductor (the limits of the
conductor may be considered an imaginary cone, having the
point of the conductor for its apex, and a base equal to twiea
tha height of the point above any particular portion of the
building), 6. No lau’ge mass«» of metal or gas-pipes should
be within striking distance of the conductor.
Round iron bars 1 inch diameter are sufficient for all
ordinary buildings. Far large lofty buildings larger conduc-
tors are required. Wire rope is being used in preference to
bar iron. Iron if used should be galvanised.
Copper is five tftiea as good a conductor as iron. The
French use copper wire ropes of from 0 • 4 to 0 • 8 inch diameter
for each 8‘i feet of height. Michel considers a galvanized iruu
rope 0*8 in diameter to be ample for most purposes.
Copper strips, No. 11 B.W.G. and 4 inches wide, are used for
conductors for factory chimneys in Lancashire. For St. Paul’s
Cathedral, 8 copper wire ropes i inch diameter are used.