722 



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 



