METALLURGY'S CONTRIBUTION TO ACCURACY IN TIMEMEASURING G . E. SXVBROOKS, HAMILTON WATCHCOMPANY, LANCASER.PENNSYLVANIA
Afterjiwe yaws of painstaking research the Hamilton Watch ComNny has introduced a notworthy imprmement in watch construction, i n the form of the Elinuar watch. The construction of the Elinwar watch d%yersfrom the older model, in that the bimetallic balance wheel and the conuentionnl steel hairspring h u e been refilaced by a monometallic balance wheel with a two-piece friction-fl balance staff and a n Elinwar hairspring. The elasticity of the conventional steel hairspring decreases as the temperature increases; in other words, the hairspring becomes weaker when warm and stronger when cool. This article giues a cross-seciion of some of lhe many factors which had to be considered before the qualities, as outlined, could be incorfiorated in a highgrade watch mmement.
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Time is so important a factor in the success of chemical processes that reliability in timepieces is a necessary characteristic of the chemist's instrumental equipment. Few chemists, however, have had occasion to acquaint themselves with the structural essentials of the small mechanism on which the accuracy of their timing depends. Fewer still, perhaps, realize how importantly metallurgy has influenced the make-up of watches. This article tells of an obstacle to correct time-measuring which remained insurmountable for centuries and was never fully overcome until metallurgy and the study of physical properties of metals revealed the way to nullify it and, by the same token, to combat other enemies of watch accuracy. Recent improvement of a tiny part of one of the smallest mechanisms marks a forward step of interest to all whose vocational activities include precise timing of processes, operations, motions. This improvement is of practical interest to every engineer and to every layman, though it especially claims the attention, perhaps, of the metallurgist, for it comprises a new use of an alloy originally developed several years ageElinvar. This, and other achievements in nickel-iron alloys, won for their originator, Dr. Charles Edouard Guillaume, the honor of being numbered among the recipients of the Nobel Award in physics. He named the new alloy from the first syllables of the two words which indicate its chief characteristimLasticity, inwar-iahle.
One Improvement Creates Others The improvement in watches and precision instruments which this alloy has made possible is not confined to the hairspring in which it is used. As a 1358
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hairspring of this metal is not affected in elasticity by atmospheric temperature variations there is no need for the compensating ha1 ance wheel heretofore used to connteract the error that would otherwise ensue with the convenUALANCE HERETOPORE IUECESFIGURE 1.-HIMETALLIC tional carbon steel SARILY USEDTO COUNTERACT EFFECTS OF TEMPERATURE hairspring. Without VARIATIONS ON THE CONVENTIONAL CARBON STEELHAIRa corrective, these SPRING oa A WATCH variations in hairA, arm. C, C, cuts in bimetallic rim to permit enlargespring e l a s t i c i t y ment or contraction of the circumference. Due to the higher coefficient of expansion of brass over steel, an inwould cause an error crease in temperature forces the free ends at C, C, toward the center of oscillation, thus reducing the weight to be in timekeeping a t the oscillated by the hair-spring whose elasticity is lessened by rate of 5 to 7 seconds the same change in temperature. Conversely, the weight a day for each degree is increased at the periphery of this balance wheel by a decrease in temperature which gives the hampring greater (F.) rise or fall of elasticity. tem~eratnre. The difficulty was fairly well (but not completely) overcome by the invention of the compensating balance by Pierre k R o y in 1766. LeRoy's compensating balance utilizes the comparative expansibility of brass and steel as the basic principle in counteracting the error which the effects of temperature changes wouldotherwise have caused. It consists of a wheel with one diametric cross arm, bored a t the center for admission of an arbor which also carries the collet to which the hairspring is fixed. The rim of the balance wheel is BALANCEWHEEL FIG- 2.-UNCUT,MONOMETALLIC oa NON-MAGNETIC METAL,USED IN CONJUNCTION WITH composed of brass ELINVAR HAIR~PRING, Wmca MAINTAINS CONSTANT the outside) and ELASTICITY TBROIJGH NORRANGE oa Arnos~m~rc TE~~~ERAT~~ES steel (on the inside),
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the arm being also of steel. The construction is clearly illustrated in Figure 1. An essential feature of this compensating balance is the cutting of the rim as shown a t C, C, in the illustration referred to. Certain practical difficultiesin maintaining the poise of the balance are due to this. The wheel is easily deformed. Besides, it has been found impossible to secure uniform compensation throughout an adopted range of temperature. This is because the elasticity of the hairspring does not vary in exactly the same ratio as the expansion and contraction of the brass and steel rim of the balance wheel. *.,. ," e.-< a s For example, a watchwith ," this type of compensating balance correct a t two tem9. perature e x t r e m e s s a y *"S
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