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The Expansion and Contraction of Balances

The American Waltham Watch Company use a simple little contrivance shown at Fig. 9 for indicating the expansion and contraction of balances. It is composed of a steel disc, on one side of which a scale is etched and opposite the scale a hole is drilled and tapped to receive the screw that holds the balance. One of the screws of the balance to be tested is removed and the indicating needle is screwed in its place. The steel disc is held by means of a pair of sliding tongs over an alcohol lamp, or can be heated in any other way, and the expansion will be indicated by the movement of the needle on the scale. Figs. 10, 11 and 12 illustrate the expansion and contraction of balances. With an increase of temperature the rim is bent inward, thus reducing the size of the balance. This is owing to the fact that brass expands more than steel, and in endeavoring to expand it, bends the rim inward. The action is, of course, reversed by lowering the temperature below normal. Some adjusters spin a balance close to the flame of a lamp before using, in order to subject it to a higher temperature than it is likely to meet in use. The balance is then placed upon a cold iron plate and afterward tested for poise. The balance is then trued if found necessary, and the operation is repeated until it is found to be in poise after heating. Sir G. B. Airy, who has devoted much time to experiments with the compensation balance, declares that when heated the loss from the weakening of the balance spring is uniformly in proportion to the increase of the temperature. The compensation balance does not meet the temperature error exactly, for with a decrease of temperature the rims expand a trifle too much and the contraction is not sufficient with the increase of temperature, and for this reason it is only possible to correct an adjustment in a watch at two points. Watches are usually adjusted at 50 deg. and 85 deg., unless they are intended for shipment to very hot climates, when a higher point is selected. Marine chronometers, however, are usually adjusted at about 45 deg. and 90 deg., but where intended for use on ships in the tropics, due allowance is made and a higher degree selected. Of course the rule is reversed where watches or chronometers are adjusted for use in the arctic regions. Under ordinary circumstances there would be a middle temperature error, with a steel hairspring, of two seconds in twenty-four hours.

In temperatures below normal the balance is larger in diameter, and the arc of vibration is less than it would be in a temperature above normal when the balance is smaller in diameter and the arc large, and its time of vibration is also effected by the hairspring so that it is not possible to predict with certainty the amount of middle temperature error. Watches are sometimes purposely left fast in the short arcs in order to lessen the middle temperature error and thereby take advantage of the above circumstances. Numerous forms of compensation balances and auxiliaries have been devised in order to try and counteract for the middle temperature error. Most of them, however, are utterly valueless. For a fuller description see Compensation Balance.