74]
To get up such an index arc we first make a stand as shown at _E F_,
Fig. 73. The arc _D_ is made to 11/2" radius, to agree with the index
hand _A'_, and is divided into twelve degree spaces, six each side of a
zero, as shown at Fig. 74, which is an enlarged view of the index _D_ in
Fig. 72. The index arc is attached to a short bit of wire extending down
into the support _E_, and made adjustable as to height by the set-screw
_l_. Let us suppose the index arc is adjusted to the index hand _A'_,
and we move the fork as suggested; you see the hand would show exactly
the arc passed through from bank to bank, and by moving the stand _E F_
we can arrange so the zero mark on the scale stands in the center of
such arc. This, of course, gives the angular motion from bank to bank.
As an experiment, let us close the bankings so they arrest the fork at
the instant the tooth drops from each pallet. If this arc is ten
degrees, the pallet action is as it should be with the majority of
modern watches.
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