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CONNECTION WITH CRUTCH
A perfectly firm suspension is essential to good
performance of any clock, and the heavier the pendulum
the firmer its support must be. It is well
known that clocks will influence each other when
fixed against a wooden wall, however firm it may
appear, and that one pendulum will thus actually
set another of equal length vibrating.
Consequently
a free pendulum can be kept swinging
from an apparently immovable support which has
an invisible vibration or twist imparted to it by a
clock below beating in the same time. All good
regulators have the pendulum cock screwed to the
back of the case, and that firmly screwed to a
strong wall; or, better still, the cock is cast with
a cast iron back or bracket which also carries the
movement, and that is screwed directly to the wall,
the whole front of the case lifting off.
In the Westminster
clock the pendulum cock is a large iron
frame built in right through the wall with a flange
behind, and the cocks of many other large clocks
are fixed with bolts through the wall. With a firm
suspension the pendulum swings farther than with
a weak one, and we shall see afterwards that (other things being equal) the
errors of clocks vary inversely as the square and sometimes as the cube of
the arc.
It is hardly necessary to say that the whole of the pendulum suspension
should be so adjusted that the bend of the spring may be exactly opposite
the pivot of the pallet arbor, in order that the fork which embraces the
pendulum, as described at page 21, may have as little friction as possible. It
must not however be tight, for the reason just now given, viz. that no point in
the pendulum describes quite a circle, and the difference is sensible enough
generally to stop the clock if the fork fits the pendulum rod too tightly.
It is equally necessary that the plane of vibration of the pendulum should
be exactly at right angles to the pallet arbor, or else there will be a sliding
motion of the pendulum backwards and forwards in the fork. But obvious as
these things are, they are all constantly neglected, especially in large clocks,
where the friction of all the parts is also the greatest.
The fork indeed is
seldom left too tight, because that mistake tells its own story directly; but,
by way of making up for it, it is very often so loose that you hear the shake
of it from one side to the other at every beat. There ought to be a drop of
oil there, as that is just enough to keep a steady hold without either shake
or tightness, since oiled surfaces are not really in metallic contact.
In old church clocks a long pendulum was often put for convenience a
good way off the clock, but so as to swing in the same plane as the crutch,
which is connected with it by a light horizontal wooden bar, no heavier than
an organ ‘tracker.’ The pendulum top too was often higher than the pallet
arbor, so that the pallets moved through a larger arc than the pendulum.
There is no advantage in that; but if the pivots at the ends of the horizontal
bar fit closely, there may be less loss of power by ‘shake’ than there generally
is between the fork and the pendulum. Indeed I have seen old regulators
made in that way, even with the pendulum in the usual place, by using two
short horizontal arms thus,
, C being the pivot in the crutch, P the
pivot in the pendulum, and D the one at the junction of the two arms DC,
DP.
But this is decidedly inferior to the other plan of putting the pendulum
on one side of the clock, since that requires only two pivots instead of three,
and there is no twisting action on the pendulum. If the horizontal bar
were attached by slight springs it would be better still, for there would then
be neither shake nor friction, and the springs would no more impede the
action than the pendulum spring does, for any moderate arc.
There is yet
in existence one great London clock of the last century with fans on the
pendulum just above the bob (like the wings on the ankles of Mercury) to
prevent it swinging too far, and they are actually placed obliquely, of which
the effect of course is to make it swing with a twist.
The late Mr. Vulliamy
told me he removed fans from the pendulum of the Horse Guards clock.
Sometimes heavy pendulums are hung by two narrow springs instead of one
PENDULUM SPRINGS. 33
broad and thin one, to secure the vibration in the proper plane; but it is a
bad plan, because it is difficult to get the two springs equal in all respects.
The springs of ‘regulator’ pendulums of 14 or 15 lbs. are generally about 1/2
an inch broad and 2 inches long; I mean clear of the chops of course; that
of the 6 cwt. pendulum at Westminster is 3 in. by 5 and 1
1/60 in. thick.
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