Figure 3 (fit. 123) shows a later form of regulation; r is the balance wheel and bb the hair-spring, whose end c of the outer coil is fastened at d The balance-arbor is held in an upright position by the "bridge" K, R, A; on the upper side of the bridge, just over the centre bearing of the balance r, a small round steel plate ee is fastened by two screws; this plate fits into the beveled edge of a steel ring (ss) which has two arms—a long pointed one (f) and a short one (g)—and the ring is held in any given position by its friction against the little steel plate ee. The long arm serves as a pointer for the graduated part RA of the bridge. On the end of the lower side of the arm g are two pins, between which the outside coil of the hair-spring is placed. If the arm f is turned toward R the hair-spring is lengthened, if turned toward A it is shortened, and there is thus produced the effect either of retarding or of accelerating the oscilla tions of the balance.
Compensating Balance.—Changes of temperature will affect the bal ance and hair-spring in the same manner as they affect the pendulum, and produce variations in their dimensions, which of course will also either retard or accelerate the vibrations ,of the balance if the temperature rises or falls. To neutralize this effect the compensating balance invented by John Harrison is employed, the principle of which is also based upon the difference in the expansions of different metals under the same tempera ture. In Figure 2 the balance is not of one continuous piece, but is cut into halves ab and a'b', each of which is fastened on one side to the arms cc, leaving the entire semicircle of each half of the balance free. Each half of the balance is made of brass and steel strips, lying parallel and fused together, as is shown in the Figure. The outer strip is of steel, and the inner strip is of brass. Since under a rising temperature the brass strips will lengthen more than the steel strips, the free ends of the balance will tend to move toward the centre, thereby making the circumference of the balance smaller; consequently it would vibrate more rapidly if at the same time the change of temperature had not also lengthened the hair-spring and thereby retarded the oscillations. A sinking of temperature will nat urally have the opposite effect and enlarge the circumference of the bal ance, thereby neutralizing the accelerating effect of the shrinking hair spring. On the periphery of the balance are screws (i), by which its vibrations may be controlled. If they are screwed more or less deeply into the balance, its circumference will be greater or smaller and the number of oscillations will be changed accordingly, thus making the arrangement for shortening or lengthening the hair-spring as described above unnecessary. This manner of adjusting the balance is generally
adopted for marine time-keepers. The adjustment of the compensation is further effected by bringing the screws i more or less near the free ends of the balance. This is the purpose of the extra screw-holes indi cated in the Figure. Moving the screws toward the free end will further the retarding tendencies of the compensation, while if screwed near to the fixed end the accelerating tendencies are effected.
The escapement serves two purposes: it controls the progress of the train and gives the necessary impulses to the regulator to continue its oscillations. The oldest escapement known is the " verge " or "crown "-wheel. We find it in the oldest clock known (see Von Wick's clock, pl. 124, 2), and it is still in use in watches, although its claim to accuracy is entitled to but little consideration as compared with the escapements of a later date. It consists of the escape- or crown-wheel (/5/. 123, fig. IT) and of the balance-staff ab, called the " verge," which has two smooth projections (1, 1'), called "pallets," at an angle of 95.02 degrees. The distance between the pallets from centre to centre is equal to the diameter of the crown-wheel. The drawing gives a view of the escapement as it is placed in a watch, and shows the verge with the hair-spring, hut omits the balance. The escape-wheel has either eleven or thirteen teeth; in the Figure it has thirteen: the escape-wheel, being impelled by a wheel of the watch-train which gears into its pinion I, moves in the direction of the arrow, but is obstructed in its progress by the pal let / at the tooth a. The tooth a presses against the pallet and turns it upward high enough to escape. By this operation the entire verge has turned and moved the pallet 1' downward, which now impedes the prog ress of the wheel at the tooth on the opposite side. The tooth will now lift the pallet 1' until it escapes also, then the tooth a drops against the pallet 1, which will repeat the same operation. In this manner the escape-wheel is alternately stopped in its progress by the two pallets /, 1', whereby it ets the verge and its balance into an oscillating motion, which is controlled by the hair-spring in a manner already demonstrated. When one tooth escapes and another drops against a pallet, the momentum of the balance will still move the pallet in a direction opposite the teeth, thereby compelling the crown-wheel to move slightly backward until the hair-spring reverses the vibration of the balance.