To demagnetize the watch would bring it back to its original condition, but a second visit to the lighting plant would again spoil its time-keeping qualities. The watchmakers now have a way of making watches so that they are not affected by magnetism, but comparatively few of the time pieces in use are non-magnetic, and the average watch is subject to these seasons of fickleness.
The exceedingly fine and exact construction of the watch is not realized by the average possessor of the article. An examination of the works of a watch shows the mechanism as now constructed, although very small in size, to be accurately planned and executed. Changes of temperature are provided for, so that the movement is automatically adjusted. The mainspring and train of gears are usually concealed, while the balance and hair springs are in full view when the case is open. Upon the regularity of the movement of the balance depends the time keeping quality of the watch. On looking closely at the balance, you will observe that it is not a complete ring, but two halves supported at one end. These rings bear a number of large-headed screws, placed at irregular distances, which give it the exact weight and balance required. These half rings will also be found, on looking closely, to be composed of two metals so closely joined that a difference in colour alone gives evidence of the quality. This arrangement of iron and brass, on account of their different coefficients of expansion and contraction with changes of temperature, has been so carefully constructed that, with changes of temperature, the balance assumes such forms as to give it a uniform rate of motion.
The parts affected by magnetism are the balance and springs. The balance in an ordinary watch moves five times a second, 18,000 times an hour, and 432,000 times each day; but a slight change in the forces that move it is necessary to make a difference of several minutes each day. As the balance moves back and forth, the magnetism of the mainspring is pulling or pushing it. If this force were constant, and always in the same direction, the watch would run uniformly. Such, however, is not the case. When the mainspring is tightly wound, its magnetic poles are in a certain direction, and in unwinding they are constantly changing, so that the direction of this force is also constantly changed. The effect on the balance is to cause the watch to run too fast sometimes, and too slow at other times.
Non-magnetic watches are made with these parts of a non-magnetic metal, so that they are not influenced by electric machinery. For testing watches a small compass is used. When placed over the balance, the needle will vibrate with the motion of the balance in proportion to its magnetism.
A BOY'S WHEEL-BARROW
Fig. 213. A boy's wheel-barrow. Perspective view
