Inasmuch as the seconds hand is to turn round sixty times while the minute hand turns round once, it is obvious that the arbor of the minute hand must be connected to the arbor of the seconds hand by a train of cogwheels so arranged as to multiply by sixty. This of course involves us in having large and small cogwheels.
Fig. 46.
The small cogwheels usually have eight teeth, and are for convenience of manufacture, as also to stand prolonged wear, cut out of the solid steel of the arbor. They are nicely polished.
The easiest pair of wheels to use will be two pinions of eight teeth, or “leaves,” as they are called, and two cogwheels, one of sixty-four teeth, the other of sixty teeth.
It is then clear that if the arbor A turns round once in an hour, the arbor B will turn round eight times in an hour, and C will turn round (60 × 64)/(8 × 8) = 60 times in an hour, or once in each minute.
By having 480 teeth on the cogwheel on A, you could, of course, make C go round once in a minute without the use of any intermediate arbor such as B.
Fig. 47.
But this would not be a very convenient plan. For as the wheel on A is usually about two and a quarter inches in diameter, to cut 480 teeth on so small a wheel would involve us in cutting about sixty teeth to the inch. The teeth would thus be microscopically small, and would have to be set so fine that the least dirt would clog them. Moreover, the pinion of eight leaves would have to be microscopic. For these reasons, therefore, it is usual in clocks not to use wheels with teeth more than sixty or sixty-four in number, and to diminish the motion gradually by means, where needful, of intermediate arbors. We have next to consider how the weight is to be arranged so as to turn the arbor A once round in an hour. We know that we have five feet of space for the weights to fall in. If we arrange to have what is called a double fall, as shown in the sketch, then, allowing room for pulley wheels, we shall find that our string may be practically about nine feet in length.