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escapement commonly used in clocks. The escape-wheel is mounted on the shaft of the last cog of the driving train, the pallet on a spindle from which depends a split arm embracing the rod and the pendulum. We must be careful to note that the pendulum _controls_ motion only; it does not cause movement.
The escape-wheel revolves in a clockwise direction. The two pallets _a_ and _b_ are so designed that only one can rest on the teeth at one time.
In the sketch the sloping end of _b_ has just been forced upwards by the pressure of a tooth. This swings the pallet and the pendulum. The momentum of the latter causes _a_ to descend, and at the instant when _b_ clears its tooth _a_ catches and holds another. The left-hand side of _a_, called the _locking-face_, is part of a circle, so that the escape-wheel is held motionless as long as it touches _a_: hence the term, "dead beat"--that is, brought to a dead stop. As the pendulum swings back, to the left, under the influence of gravity, _a_ is raised and frees the tooth. The wheel jerks round, and another tooth is caught by the locking-face of _b_. Again the pendulum swings to the right, and the sloping end of _b_ is pushed up once more, giving the pendulum fresh impetus. This process repeats itself as long as the driving power lasts--for weeks, months, or years, as the case may be, and the mechanism continues to be in good working order.
Metal expands when heated; therefore a steel pendulum which is of the exact length to govern a clock correctly at a temperature of 60 would become too long at 80, and slow the clock, and too short at 40, and cause it to gain. In common clocks the pendulum rod is often made of wood, which maintains an almost constant length at all ordinary temperatures. But for very accurate clocks something more efficient is required. Graham, the partner of Thomas Tompion, took advantage of the fact that different kinds of metal have different ratios of expansion to produce a _self-compensating_ pendulum on the principle ill.u.s.trated by Fig. 203. He used steel for the rod, and formed the _bob_, or weighted end, of a gla.s.s jar containing mercury held in a stirrup; the mercury being of such a height that, as the pendulum rod lengthened with a rise of temperature, the mercury expanded _upwards_ sufficiently to keep the distance between the point of suspension and the centre of gravity of the bob always the same. With a fall of temperature the rod shortened, while the mercury sank in the jar. This device has not been improved upon, and is still used in observatories and other places where timekeepers of extreme precision are required. The milled nut S in Fig.
203 is fitted at the end of the pendulum rod to permit the exact adjustment of the pendulum's length.
For watches, chronometers, and small clocks
THE SPRING BALANCE
takes the place of the pendulum. We still have an escape-wheel with teeth of a suitable shape to give impulses to the controlling agent.
There are two forms of spring escapement, but as both employ a hairspring and balance-wheel we will glance at these before going further.
[Ill.u.s.tration: FIG. 203.]
The _hairspring_ is made of very fine steel ribbon, tempered to extreme elasticity, and shaped to a spiral. The inner end is attached to the arbor of the _balance-wheel_, the outer end to a stud projecting from the plate of the watch. When the balance-wheel, impelled by the escapement, rotates, it winds up the spring. The energy thus stored helps the wheel to revolve the other way during the locking of a tooth of the escape-wheel. The time occupied by the winding and the unwinding depends upon the length of the spring. The strength of the impulse makes no difference. A strong impulse causes the spring to coil itself up more than a weak impulse would; but inasmuch as more energy is stored the process of unwinding is hastened. To put the matter very simply--a strong impulse moves the balance-wheel further, but rotates it quickly; a weak impulse moves it a shorter distance, but rotates it slowly. In fact, the principle of the pendulum is also that of the hairspring; and the duration of a vibration depends on the length of the rod in the one case, and of the spring in the other.
Motion is transmitted to the balance by one of two methods. Either (1) directly, by a cylinder escapement; or (2) indirectly, through a lever.
[Ill.u.s.tration: FIG. 204.--"Cylinder" watch escapement.]
THE CYLINDER ESCAPEMENT
is seen in Fig. 204. The escape-wheel has sharp teeth set on stalks.
(One tooth is removed to show the stalk.) The balance-wheel is mounted on a small steel cylinder, with part of the circ.u.mference cut away at the level of the teeth, so that if seen from above it would appear like _a_ in our ill.u.s.tration. A tooth is just beginning to shove its point under the nearer edge of the opening. As it is forced forwards, _b_ is revolved in a clockwise direction, winding up the hairspring. When the tooth has pa.s.sed the nearer edge it flies forward, striking the inside of the further wall of the cylinder, which holds it while the spring uncoils. The tooth now pushes its way past the other edge, accelerating the unwinding, and, as it escapes, the next tooth jumps forward and is arrested by the outside of the cylinder. The balance now reverses its motion, is helped by the tooth, is wound up, locks the tooth, and so on.
THE LEVER ESCAPEMENT
is somewhat more complicated. The escape-wheel teeth are locked and unlocked by the pallets P P^1 projecting from a lever which moves on a pivot (Fig. 205). The end of the lever is forked, and has a square notch in it. On the arbor of the balance-wheel is a roller, or plate, R, which carries a small pin, I. Two pins, B B, projecting from the plate of the watch prevent the lever moving too far. We must further notice the little pin C on the lever, and a notch in the edge of the roller.
[Ill.u.s.tration: FIG. 205.--"Lever" watch escapement.]
In the ill.u.s.tration a tooth has just pa.s.sed under the "impulse face" _b_ of P^1. The lever has been moved upwards at the right end; and its forked end has given an impulse to R, and through it to the balance-wheel. The spring winds up. The pin C prevents the lever dropping, because it no longer has the notch opposite to it, but presses on the circ.u.mference of R. As the spring unwinds it strikes the lever at the moment when the notch and C are opposite. The lever is knocked downwards, and the tooth, which had been arrested by the locking-face _a_ of pallet P, now presses on the impulse face _b_, forcing the left end of the lever up. The impulse pin I receives a blow, a.s.sisting the unwinding of the spring, and C again locks the lever. The same thing is repeated in alternate directions over and over again.
The watchmaker has had to overcome the same difficulty as the clockmaker with regard to the expansion of the metal in the controlling agent. When a metal wheel is heated its spokes lengthen, and the rim recedes from the centre. Now, let us suppose that we have two rods of equal weight, one three feet long, the other six feet long. To an end of each we fasten a 2-lb. weight. We shall find it much easier to wave the shorter rod backwards and forwards quickly than the other. Why? Because the weight of the longer rod has more leverage over the hand than has that of the shorter rod. Similarly, if, while the ma.s.s of the rim of a wheel remains constant, the length of the spokes varies, the effort needed to rotate the wheel to and fro at a constant rate must vary also. Graham got over the difficulty with a rod by means of the compensating pendulum. Thomas Earnshaw mastered it in wheels by means of the _compensating balance_, using the same principle--namely, the unequal expansion of different metals. Any one who owns a compensated watch will see, on stopping the tiny fly-wheel, that it has two spokes (Fig. 206), each carrying an almost complete semicircle of rim attached to it. A close examination shows that the rim is compounded of an outer strip of bra.s.s welded to an inner lining of steel. The bra.s.s element expands more with heat and contracts more with cold than steel; so that when the spokes become elongated by a rise of temperature, the pieces bend inwards at their free ends (Fig. 207); if the temperature falls, the spokes are shortened, and the rim pieces bend outwards (Fig. 208).
This ingenious contrivance keeps the leverage of the rim constant within very fine limits. The screws S S are inserted in the rim to balance it correctly, and very fine adjustment is made by means of the four tiny weights W W. In ships' chronometers, the rim pieces are _sub_-compensated towards their free ends to counteract slight errors in the primary compensation. So delicate is the compensation that a daily loss or gain of only half a second is often the limit of error.
[Ill.u.s.tration: FIG. 206. FIG. 207. FIG. 208. A "compensating" watch balance, at normal, super-normal, and sub-normal temperatures.]
KEYLESS WINDING MECHANISM FOR WATCHES.
The inconvenience attaching to a key-wound watch caused the Swiss manufacturers to put on the market, in 1851, watches which dispensed with a separate key. Those of our readers who carry keyless watches will be interested to learn how the winding and setting of the hands is effected by the little serrated k.n.o.b enclosed inside the pendant ring.
There are two forms of "going-barrel" keyless mechanism--(1) The rocking bar; (2) the shifting sleeve. The _rocking bar_ device is shown in Figs.
209, 210. The milled head M turns a cog, G, which is always in gear with a cog, F. This cog gears with two others, A and B, mounted at each end of the rocker R, which moves on pivot S. A spring, S P, attached to the watch plate presses against a small stud on the rocking bar, and keeps A normally in gear with C, mounted on the arbor of the mainspring.
[Ill.u.s.tration: FIG. 209.--The winding mechanism of a keyless watch.]
To wind the watch, M is turned so as to give F an anti-clockwise motion.
The teeth of F now press A downwards and keep it in gear with C while the winding is done. A spring click (marked solid black) prevents the spring uncoiling (Fig. 209). If F is turned in a clockwise direction it lifts A and prevents it biting the teeth of C, and no strain is thrown on C.
To set the hands, the little push-piece P is pressed inwards by the thumb (Fig. 210) so as to depress the right-hand end of R and bring B into gear with D, which in turn moves E, mounted on the end of the minute-hand shaft. The hands can now be moved in either direction by turning M. On releasing the push-piece the winding-wheels engage again.
The _shifting sleeve_ mechanism has a bevel pinion in the place of G (Fig. 209) gearing with the mainspring cog. The shaft of the k.n.o.b M is round where it pa.s.ses through the bevel and can turn freely inside it, but is square below. On the square part is mounted a little sliding clutch with teeth on the top corresponding with the other teeth on the under side of the bevel-wheel, and teeth similar to those of G (Fig.
209) at the end. The clutch has a groove cut in the circ.u.mference, and in this lies the end of a spring lever which can be depressed by the push-piece. The mechanism much resembles on a small scale the motor car changing gear (Fig. 49). Normally, the clutch is pushed up the square part of the k.n.o.b shaft by the spring so as to engage with the bevel and the winding-wheels. On depressing the clutch by means of the push-piece it gears with the minute-hand pinion, and lets go of the bevel.
[Ill.u.s.tration: FIG. 210.--The hand-setting mechanism in action.]
In one form of this mechanism the push-piece is dispensed with, and the minute-wheel pinion is engaged by pulling the k.n.o.b upwards.
THE HOUR-HAND TRAIN.
[Ill.u.s.tration: FIG. 211.--The hour-hand train of a clock.]
The teeth of the mainspring drum gear with a cog on the minute-hand shaft, which also carries one of the cogs of the escapement train. The shaft is permitted by the escapement to revolve once an hour. Fig. 211 shows diagrammatically how this is managed. The hour-hand shaft A (solid black) can be moved round inside the cog B, driven by the mainspring drum. It carries a cog, C. This gears with a cog, D, having three times as many teeth. The cog E, united to D, drives cog F, having four times as many teeth as E. To F is attached the collar G of the hour-hand. F and G revolve outside the minute-hand shaft. On turning A, C turns D and E, E turns F and the hour-hand, which revolves 1/3 of 1/4 = 1/12 as fast as A.
On these unfortunately necessary mechanisms a great deal of ingenuity has been expended. With the advance of luxury and the increased worship of wealth, it becomes more and more necessary to guard one's belongings against the less scrupulous members of society.
[Ill.u.s.tration: FIG. 212.]
The simplest form of lock, such as is found in desks and very cheap articles, works on the principle shown in Fig. 212. The bolt is split at the rear, and the upper part bent upwards to form a spring. The under edge has two notches cut in it, separated by a curved excrescence. The key merely presses the bolt upwards against the spring, until the notch, engaging with the frame, moves it backwards or forwards until the spring drives the tail down into the other notch. This primitive device affords, of course, very little security. An advance is seen in the
[Ill.u.s.tration: FIG. 213.]
The bolt now can move only in a horizontal direction. It has an opening cut in it with two notches (Figs. 213, 214). Behind the bolt lies the _tumbler_ T (indicated by the dotted line), pivoted at the angle on a pin. From the face of the tumbler a stud, S, projects through the hole in the bolt. This stud is forced into one or other of the notches by the spring, S^1, which presses on the tail of the tumbler.
[Ill.u.s.tration: FIG. 214.]
In Fig. 213 the key is about to actuate the locking mechanism. The next diagram (Fig. 214) shows how the key, as it enters the notch on the lower side of the bolt to move it along, also raises the tumbler stud clear of the projection between the two notches. By the time that the bolt has been fully "shot," the key leaves the under notch and allows the tumbler stud to fall into the rear locking-notch.
A lock of this type also can be picked very easily, as the picker has merely to lift the tumbler and move the bolt along. Barron's lock, patented in 1778, had two tumblers and two studs; and the opening in the bolt had notches at the top as well as at the bottom (Fig. 215). This made it necessary for both tumblers to be raised simultaneously to exactly the right height. If either was not lifted sufficiently, a stud could not clear its bottom notch; if either rose too far, it engaged an upper notch. The chances therefore were greatly against a wrong key turning the lock.