305
OF ENGINEERING F0RMULÆ.
Construction of Electromagnets.
Let a = thickness of magnetizing helix.
6 = total length of the two bobbins.
c = diameter of core.
g = diameter of covered wire in metres.
f = constant, varying generally from 1’4
with fine wires to 1'2 with mode-
‘ rately coarse wires, but dependent on
the nature of the covering. .
A = attractive force in fundamental units.
P = attractive force in grammes at 1 mm.
distance.
E = electromotive force of current in Daniell’s
cells.
II = length of magnetizing helix, in metres.
I = quantity of current, measured as E
acting on m metres of telegraph wire
of 4 mm. diameter; or as hundredths
of a weber; or as in G. M. S. units.
t = number of convolutions.
R = resistance in metres of telegraph wire of
exterior circuit, including that of the
source of electricity (100 metres =
1 ohm).
F = magnetic moment,
conductivity of R .
a conductivity of H .
b 0 • 1722 when E is in Daniell cells and
R is in metres of telegraph wire;
= • 015957 when E and R are in B. A.
units.
h = number of cells of battery
g = e. m. f. of each cell.
p = resistance of each cell.
x