Part 26 (1/2)

escapement commonly used in clocks The escape-wheel istrain, the pallet on a spindle fro 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 ti 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 heldas it touches _a_: hence the terht to a dead stop As the penduluravity, _a_ is raised and frees the tooth The wheel jerks round, and another tooth is caught by the locking-face of _b_ Again the pendulu end of _b_ is pushed up oncethe pendulu as the driving power lasts--for weeks, months, or years, as the caseorder

COMPENSATING PENDULUMS

Metal expands when heated; therefore a steel penduluovern a clock correctly at a te at 80, and slow the clock, and too short at 40, and cause it to gain In common clocks the pendulum rod is often th at all ordinary te more efficient is required Grahae of the fact that different kinds of metal have different ratios of expansion to produce a _self-co 203 He used steel for the rod, and for ht that, as the penduluthened 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 extre

203 is fitted at the end of the penduluth

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 ient

There are two for and balance-wheel ill glance at these before going further

[Illustration: 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, i The energy thus stored helps the wheel to revolve the other way during the locking of a tooth of the escape-wheel The ti depends upon the length of the spring The strength of the i to coil itself up y is stored the process of unwinding is hastened To put theimpulse 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 pendulu; 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 escapeh a lever

[Illustration: FIG 204--”Cylinder” watch escape 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 circumference cut away at the level of the teeth, so that if seen from above it would appear like _a_ in our illustration 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 passed 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 P1 projecting fro 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 s fro too far We must further notice the little pin C on the lever, and a notch in the edge of the roller

[Illustration: FIG 205--”Lever” watch escapement]

In the illustration a tooth has just passed under the ”impulse face” _b_ of P1 The lever has been iven an i winds up The pin C prevents the lever dropping, because it no longer has the notch opposite to it, but presses on the circu unwinds it strikes the lever at the moment when the notch and C are opposite The lever is knocked doards, and the tooth, which had been arrested by the locking-face _a_ of pallet P, now presses on the i the left end of the lever up The i of the spring, and C again locks the lever The saain

COMPENSATING BALANCE-WHEELS

The watchmaker has had to overcoard to the expansion of the ent When a then, 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 e over the hand than has that of the shorter rod Similarly, if, while the th of the spokes varies, the effort needed to rotate the wheel to and fro at a constant rate ot over the difficulty with a rod bypendulum Thomas Earnshawbalance_, using the same principle--namely, the unequal expansion of different metals Any one ns a co 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 brass welded to an inner lining of steel The brass element expands more with heat and contracts more with cold than steel; so that when the spokes becoated by a rise of te 207); if the temperature falls, the spokes are shortened, and the rienious 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 adjusthts W W In shi+ps' chronometers,[40] the rim pieces are _sub_-coht errors in the primary compensation So delicate is the coain of only half a second is often the limit of error

[Illustration: FIG 206 FIG 207 FIG 208 A ”co” watch balance, at normal, super-normal, and sub-normal temperatures]

KEYLESS WINDING MECHANISM FOR WATCHES