WATCH, a small portable machine for measuring time, the construction of which is essentially the same as that of a clock (see HonoLooy) except that the moving power is obtained from the elastic force of a coiled spring instead of from a weight, and the movement regulated, so as to be isochronous, by a balance and balance-spring (q.v.) in stead of a pendulum. The going part of a watch consists of a train of wheels and pin ions, kept in motion by a spring, called the the last and fastest wheel of g the train, the scape-wheel or balance-wheel, actin so as to keep in vibratory motion a balance whose movement, again—which is made isochronousby the action of another spring called the balance-spring—regulates to a uniform rate the revolution of the scape wheel, and consequently the motion of the rest of the train, and the uncoiling of the main-spring.
The main-spring is a thin ribbon of steel coiled in a barrel. The inner end of it is fixed to a strong spindle, the axis or arbor of the barrel, around which it is coiled, and the outer end is fixed to the inside of the barrel. By its tendency to uncoil itself, the spring sets the barrel in motion, and it produces as natny revolutions of the barrel as it makes turns itself in unwinding. As its elastic force is greater when it is tightly coiled than when it has to some extent unwound itself, the spring, if its force were applied without modification to the watch train, would act upon it unequally;the power ex erted diminishing as the spring uncoiled; so much so, that the watch could not go uni formly throughout the day, though it might keep time from one day to another. A piece of machinery, called a fusee, is employed to correct the variations in the force of the spring, and equalize the power exerted upon the train. The fusee is a cone with a spiral groove, connected with the barrel which contains the main-spring by a chain, one end of which is fixed at the broadest part of the cone, and the other end to the barrel. The barrel moves the fusee by means of the chain, which, as it runs off the sides of the fusee, is coiled upon the outside of the barrel. In winding a watch the key is placed on the axis of the fusee, and by the same movement the main-spring is coiled around its spindle, and the chain wound off the barrel, to cover the cone of the fusee. So when the spring is all coiled up, and its force upon the barrel is greatest, the chain is acting at the small end of the fusee, and its leverage upon the fusee is least; as the force of the spring diminishes, the chain having got to a broader part of the fusee, the leverage is increased; and the grooving of the fusee being, when perfect, arranged so that a section of the fusee along its a_V__ would present two hyperbolas placed back to back, secures that the force of the spring, modified by the leverage of the chain, shall produce a uni form motion of the fusee. From the fusee this motion is the watch train, the first wheel of the train—called the fusee-wheel or the °Teat wheel—being set upon the fusee. The fusee is introduced in almost all English watches; but a great
proportion of foreign watches, and most French spring clocks, have no fusee, and have the great wheel fixed on to the barrel. Accurate time-keeping is not to be looked for from such clocks or watches; but it is said that many of the main-springs made upon the continent are so skillfully contrived, that the force is pretty constant during the whole time of unwinding.
Between the train of wheels and pinions in a watch and that of a clock, until we come to the escapement, there is no difference, except that there is one more wheel and pinion in the watch-train than in the clock-train; the reason of which is, that the scape-wheel of a watch revolves, not like that of a clock, in a Minute, but usually in about six seconds, making necessary an additional wheel to revolve in a minute and carry the seconds hand. A great variety of watch escapements are in use. The oldest, which is now going out of use, is the vertical escapement. It exactly corresponds to the crown-wheel escapement in clocks (see HOROLOGY). The accompanying figure shows a watch-train with this escapement. It may be useful also as indicating, in a general way, the arrangement of the wheel-work in a watch (fig. I). The main-spring contained in the barrel B, sets in motion the barrel, which, by means of the chain c, moves at a uniform rate the fusee F, along with which turns the fuse( -wheel W, the first or great wheel of the watch-train. It will be easily seen how, from the great wheel, mo tion is communicated successively to the center-pinion D, and the center-wheel D' (which turn in an hour); to the third-wheel pinion, E, and the third wheel, which is upon the same arbor, E'; and to the fourth or contrate-wheel pinion G, and the contrate-wheel G'. The uptight teeth of the last named wheel move the balance-wheel pinion II, and with it the balance-wheel or scape wheel H', which is fixed upon its arbor. The scape-wheel (and in this escapement the contrate-wheel also) is what is called, from its shape, a crown-wheel. Upon the arbor or verge of the balance K, are two pallets, p, p, at a distance from each other equal to the diameter of the scape-wheel, and so placed that, as the scape-wheel revolves, its teeth give them alternately an impulse in different directions, which keeps up the vibra tory motion of the balance. The balance is made to vibrate isochronously by the action of the (q.v.); and its vibration regulates the escape of the teeth of the scape-wheel, and so the motion of the whole train, exactly as that of the pendulum does in an ordinary clock. The vertical escapement is liable, though in a less degree, to the same objection as the old crown-wheel and the crutch or anchor escapements in clocks. There is a recoil of the scape-wheel after one of its teeth has been stopped by a pallet, which interferes more or less with the accuracy and uniformity of the motion of the train. See HOROLOGY.