Fig. 201.
A string of catgut (or a very fine chain) is connected at one end to the circumference of the drum, and wound round it, the other end being fixed to the larger end of the fusee, which is attached to the driving-wheel of the watch or clock by the intervention of a ratchet and click (not shown). To wind the spring the fusee is turned backward by means of a key applied to the square end A of the fusee arbor, and this draws the string from off the drum on to the fusee. The force of the spring causes the fusee to rotate by pulling the string off it, coil by coil, and so drives the train of wheels. But while the mainspring, when fully wound, turns the fusee by uncoiling the string from the smallest part of the fusee, it gets the advantage of the larger radius as its energy becomes lessened.
The fusee is still used for marine chronometers, for some clocks that have a mainspring and pendulum, and occasionally for watches. In the latter it has been rendered unnecessary by the introduction of the going-barrel by Swiss watchmakers, who formed teeth on the edge of the mainspring barrel to drive the train of wheels. This kind of drum is called "going" because it drives the watch during the operation of winding, which is performed by rotating the drum arbor to which the inner end of the spring is attached. A ratchet prevents the arbor from being turned backwards by the spring. The adoption of the going-barrel has been made satisfactory by the improvements in the various escapement actions.
THE ESCAPEMENT.
Fig. 202.
The spring or weight transmits its power through a train of cogs to the escapement, or device for regulating the rate at which the wheels are to revolve. In clocks a pendulum is generally used as the controlling agent. Galileo, when a student at Pisa, noticed that certain hanging lamps in the cathedral there swung on their cords at an equal rate; and on investigation he discovered the principle that the shorter a pendulum is the more quickly will it swing to and fro. As has already been observed, Huygens first applied the principle to the governing of clocks. In Fig. 202 we have a simple representation of the "dead-beat" 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.
COMPENSATING PENDULUMS.