77. The idea with this form of index hand is, the
bent-down jaws _B'_, Fig. 77, grasp the fork as close to the pallet
staff as possible, making an allowance for the acting center by so
placing the index arc that the hand _A_ will read correctly on the index
_D_. Suppose, for instance, we place the jaws _B'_ inside the pallet
staff, we then place the index arc so the hand reads to the arc
indicated by the dotted arc _m_, Fig. 78, and if set outside of the
pallet staff, read by the arc _o_.
[Illustration: Fig. 78]
HOW A BALANCE CONTROLS THE TIMEKEEPING OF A WATCH.
We think a majority of the fine lever escapements made abroad in this
day have what is termed double-roller safety action. The chief gains to
be derived from this form of safety action are: (1) Reducing the arc of
fork and roller action; (2) reducing the friction of the guard point to
a minimum. While it is entirely practicable to use a table roller for
holding the jewel pin with a double-roller action, still a departure
from that form is desirable, both for looks and because as much of the
aggregate weight of a balance should be kept as far from the axis of
rotation as possible.
Pages:
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145