It controls the escapement which brings the whole mechanism of the clock to a standstill five times each second—in fact it is the pacemaker of the watch, for it gives the watch a step-by-step movement and fixes the rate at which the steps are taken.
FIG. 27.—ESCAPEMENT OF A WATCH
The last wheel of the watch train is what is known as an escape wheel. It is formed with teeth of an odd shape, such as shown at A in Figure 27. These teeth are engaged by a pair of pallets B and C, carried by a three-armed lever D. The pallets are usually bits of sapphire or similar hard stone to prevent wear. The third of the lever is slotted at its extremity to engage a sapphire pin E, carried by a disk F, which is mounted on the staff of the balance wheel. The escape wheel A revolves in the direction of the arrow, being impelled by the mainspring acting through the train of gears. One of the teeth of this wheel engages the pallet B, causing the lever D to swing on its axis and push the sapphire pin E toward the left, thereby giving the disk F an impulse in the same direction. Here a delicate coil spring, known as the hairspring, comes into play. Without the hairspring the parts would stand still, the escape wheel being blocked by the pallet B. The hairspring is attached at one end to the shaft or staff of the disk F and the other to the frame of the watch. It tries to hold the disk F in a fixed position, but is disturbed by the action of the escape wheel and is constantly oscillating the disk in its effort to bring it back to its normal position. When the disk swings over to the left the pallet B is clear of the teeth of the escape wheel. This releases the escape wheel and it springs forward in the direction of the arrow, but before it can move through an interval of one tooth it is arrested by the second pallet C, which has been projected into its path by the swing of the lever D. The lever swings back until the pallet C clears the escape wheel and the pallet B engages the next tooth. And so the action continues, the lever swinging back and forth and at each complete oscillation releasing one tooth of the escape wheel.
The hairspring takes up the shock of this intermittent motion and a balance wheel carried by the staff to which F is fastened steadies the oscillatory motion of the lever D. A watch is full of microscopic parts. In a small timepiece there are machine-made screws so small that without the aid of a magnifying glass one cannot see the screw threads cut upon them. But the most marvelous part of the whole watch is the delicate hairspring and the means of adjusting its tension and compensating for its expansion and contraction with changes of temperature.
INANIMATE MATTER IN CONTINUAL MOTION
When working with minute intervals of time many factors must be considered which are not even thought of in machines of grosser proportions. It never occurs to the man in the street that not only the animate world but the inanimate as well is in ceaseless and variable motion. If our eyes were capable of taking in minute microscopic details, we should see that everything is expanding or contracting, swelling or shriveling, twisting and warping in response to the atmospheric changes. Our steel bridges and skyscrapers are in constant motion; solid concrete dams must be provided with expansion joints; the Washington Monument goes through a diurnal gyration in response to the sun’s rays. Of course all this motion is almost immeasurably small. A bar of steel a mile long will expand ⅖ of an inch for every increase of a degree Fahrenheit in temperature. The expansion of a hairspring, which may be nine or ten inches long, is infinitesimally small and yet this must be considered by the watchmaker. We must remember that the escapement mechanism divides the day into 432,000 parts, each of which contains some minute error, for absolute perfection is impossible, and if we add up all these 432,000 errors they must not foot up to more than a second per day. If the hairspring expands ever so slightly its power is weakened, but this loss of power is compensated by an ingenious form of balance wheel. The rim is in two parts, half of it being attached to one spoke of the wheel and the other half to the other, as shown in Figure 28. Each half rim is formed of two strips of metal, an inner strip of steel, and an outer strip of brass fused together. Brass expands and contracts almost twice as much as steel, and hence when there is a rise of temperature the rim sections tend to curl in, bringing their center of gravity nearer the center of the wheel and making less of a load for the weakened hairspring to move, while on the other hand, when the spring is contracted by cold, the rims spread out slightly, giving it a greater load to oscillate. The weight of the balance wheel is thus automatically adjusted against variations in power of the spring.
