We will now resume the consideration of the device for measuring the extent of the angular motion of the fork and pallets. Now, before we take this matter up in detail we wish to say, or rather repeat what we have said before, which is to the effect that ten degrees of fork and lever action is not imperative, as we can get just as sound an action and precisely as good results with nine and a half or even nine degrees as with ten, if other acting parts are in unison with such an arc of angular motion. The chief use of such an angle-measuring device is to aid in comparing the relative action of the several parts with a known standard.
For use with full-plate movements about the best plan is a spring clip or clasp to embrace the pallet staff below the pallets. We show at Fig. 71 such a device. To make it, take a rather large size of sewing needle--the kind known as a milliner's needle is about the best. The diameter of the needle should be about No. 2, so that at _b_ we can drill and put in a small screw. It is important that the whole affair should be very light. The length of the needle should be about 1-5/8", in order that from the notch _a_ to the end of the needle _A'_ should be 1½". The needle should be annealed and flattened a little, to give a pretty good grasp to the notch _a_ on the pallet staff.
Good judgment is important in making this clamp, as it is nearly impossible to give exact measurements. About 1/40" in width when seen in the direction of the arrow _j_ will be found to be about the right width. The spring _B_ can be made of a bit of mainspring, annealed and filed down to agree in width with the part _A_. In connection with the device shown at Fig. 71 we need a movement-holder to hold the movement as nearly a constant height as possible above the bench. The idea is, when the clamp _A B_ is slipped on the pallet staff the index hand _A'_ will extend outward, as shown in Fig. 72, where the circle _C_ is supposed to represent the top plate of a watch, and _A'_ the index hand.
Fig. 72 is supposed to be seen from above. It is evident that if we remove the balance from the movement shown at _C_, leaving power on the train, and with an oiling tool or hair broach move the lever back and forth, the index hand _A'_ will show in a magnified manner the angular motion of the lever. Now if we provide an index arc, as shown at _D_, we can measure the extent of such motion from bank to bank.