106, which is a view of Fig. 105 seen in
the direction of the arrow _i_. This wire _k_ enters a base shown at
_D E_, Fig. 106, which is provided with a set-screw at _j_ for holding
the index arc at the proper height to coincide with the hand _B_.
[Illustration: Fig. 105]
[Illustration: Fig. 106]
A good way to get up the parts shown in Fig. 106 is to take a disk of
thick sheet brass about 1" in diameter and insert in it a piece of brass
wire about 1/4" diameter and 3/8" long, through which drill axially a
hole to receive the wire _k_. After the jaws _B''_ are clamped on the
pallet staff, we set the index arc _C_ so the hand _B'_ will indicate
the angular motion of the pallet staff. By placing the index hand _B_
on the balance staff we can get at the exact angular duration of the
engagement of the jewel pin in the fork.
Of course, it is understood that this instrument will also measure the
angles of impulse and lock. Thus, suppose the entire angular motion of
the lever from bank to bank is ten degrees; to determine how much of
this is lock and how much impulse, we set the index arc _C_ so that the
hand _B'_ marks ten degrees for the entire motion of the fork, and when
the escapement is locked we move the fork from its bank and notice by
the arc _C_ how many degrees the hand indicated before it passed of its
own accord to the opposite bank.
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