Page:Popular Science Monthly Volume 90.djvu/738

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��Popular Science Monthly

��rest of the watch simply "serves" the balance by keeping it going, and by making its rate of going conveniently visible.

You can see the hairspring in your watch vibrating just above the balance wheel. The hairspring is properly considered to be a part of the balance. Its beautiful action, as the coils expand and contract with the alternate swings of the balance, has been termed, and very aptly likened to, "breathing."

This hairspring is a vitally important part of the modern watch balance, and provides the wonderful control known as isochronism (equal timing), which over- comes the effects of variations of the main- spring's power. For the power applied to the balance varies. It is greater just after winding the watch and gradually becomes less as the mainspring runs down. As the power decreases, the balance naturally takes shorter swings. The shorter swings, one would assume, would occupy less time. Hence the watch would run faster and faster towards the end of its day's run. This would certainly be the case were it not for the isochronal control which the hair- spring exerts on the rate of speed at which the balance moves during each swing.

Making a Watch Run Uniformly

Let us assume, in order to illustrate the principle, that the balance makes one complete turn at each "journey" when the watch is wound up, but as it runs down, the lessening power produces shorter and shorter journeys of the balance until they are reduced, let us say, to only half a turn. These half-turn journeys would each require only half the time which was occu- pied in making full turns, and the watch would go twice as fast as when the balance was making journeys of one full turn each.

But the isochronal control exerted by the hairspring on the balance steps in here, and interferes with natural conditions to such extent that the balance will make the short journey at a proportionately slower speed —

���The jewels of the watch are held in place by screws

Some watches have all the pivot bearings jeweled; some just a part of them. There are watches of obscure origin with jewels in them inferior to plain drilled metal bearings. Of such watches it may be said that the more jewels the poorer the watch. It is of prime importance to have com- petent advice when selecting a watch. The cost of the movement is a measure of its efficiency

��one-half as fast — so that the short journey will require just as much time to make as the long journey.

We have seen what a great service the hairspring renders the balance in controlling its motion. Now it will be shown that the balance is not an ungrateful creature, and that it helps the hairspring quite as much as the hairspring helps it! In fact, good time- keeping is possible only because balance and hairspring cooperate in correcting in each other certain mechanical effects which need to be counteracted.

Why Your Watch Keeps Good Time in Winter as Well as in Summer

The hairspring's shortcoming is that the condition of its metal changes with varying temperatures; in cold weather it becomes stiffer, and in warm weather more pliant. This would naturally cause the watch to run fast in cold, and slow in warm weather. And that is how watches did run before the invention of the modern compensating balance. This compensating balance is cleverly devised to alter its diameter at every change of temperature. On a cold day the balance automatically grows larger and runs slower, just enough to compensate for the quickening of the hairspring's action. If the watch is then carried into a warm room, the balance promptly makes it- self smaller and runs faster by just so much as the warmed hair- spring would have made it run slower.

The balance rim is made of two metals (brass and steel) fused together. When heated or cooled, the brass (on the outside of the rim) expands or contracts faster than the steel portion forming the inside of the rim. When heat- ed , the brass, in trying to lengthen out beyond the steel, bends the arms inward, and when cooled , the brass, in trying to contract shorter than the steel, bends the arms outward.

Adjustment of a watch to heat and cold consists in altering the positions and weights of the screws on the balance rim until the action of the balance corresponds

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