:
A PRACTICAL COURSE IN HOROLOGY
26
the center pinion, the barrel 1.
is
c,
6.5
c
36
The
inside diameter of
12 millimeters.
—=— = 12
2.
has 12 leaves.
X
1
=6.5
hours,
= 5.5 turns to wind sprmg.
6.5
12
= .96
3.
12.5 •96 4.
= .17 mm., thickness of... the spnng.
5.5 It
should be understood that the answers are only ap-
proximate. In some cases, as in a a weaker spring
may
fine,
21- or 23- jewel watch,
be needed, whereas a 7- jewel watch
may
require a stronger spring.
Calculating the correct length of the mainspring. The proper length of the spring need not be calculated in so inches.
we may
Instead,
state that the spring should
many
occupy
one half of the area between the inside wall of the barrel
and the periphery of the arbor. If the spring does length
this,
the
is correct.
Observe that we say area and not space. The term space
would lead one
to infer that radial
measurements are
in-
tended, which would be incorrect, inasmuch as a spring
wound up would occupy more spring run down.
radial space than that of a
Thus, the statement often found in older
books on horology to the
effect that
we
allow one third of
the space for the arbor, one third for the space, and one third for the spring
is
slightly in error
a few coils too long.
The
correct rule should read
and indicative of a spring