337
OF ENGINEERING FORMULÆ.
Electrical Formula:, by Paget Higgs, LL.D., C.E.,
and R. 8. Brough—continued.
one square millimetre in section weighs about 8 • 5
grammes; and its resistance is about 0’20G ohm
at 0° O., and increases about 0'04 per cent, per
1° 0.
The resistance of a pri&m of German silver one
metre long and weighing one gramme is about
1'75 ohnas at 0° C.
24. The diameter d of any wire, whose specific
gravity is s, and which weighs w grammes per
metre, is
/ to
d = 1 • 12865 V - millimetres,
s ,
and its sectional area a is
a — - square millimetres.
25. The length of wire on a circular bobbin is
L = ^<A2-«2)’
where d = diameter of wire, including silk cover-
in»; b = length of bobbin; and A = outer dia-
meter and a — inner diameter of bobbin.
Winding coils: Let c = specific conductivity of
wire, that of pure copper being unity at 0° C.; I =
average length of convolution in inches; m - area
in squaw inches of ti soini-soction of tlæ coil (tlitxt
is the sectional area of the space to be filled with
wire); s = thickness in inches of insulatin'' cover-
ing; G = resistance of coil in ohms; r = radius