How pocket watches are adjusted for temperature and timing
by W. H. Samelius
editor's note: Replying to a question by "W.A.", Mr. Information, Please" has written a techical article of practical value to every serious watch repairman.
Speaking generally, a proportion of 2-fifths steel and 3-fifths brass have been found well adapted for this purpose.
The relative proportions between the width and the thickness of the rim is a matter of importance. The thinner the rim in proportion to its width, the more active it will be and the greater it its deflection due to changes in temperature. Also, the deflection by centrifugal force will be greater as will be the danger of losing the true form by slight jars or by careless handling. It is also evident that in order to produce a total given weight, a thin rim must have a greater weight in screws than a thicker one. A rim with a thickness of 36 per cent of the width has been found about right. There are four screws located about 90° apart (two at the ends of the arms, and two halfway between them on each rim) which are used to determine the practical working diameter of the balance. These are called "timing" screws and should never be changed unless to slow the balance or to increase its speed. The other screws are simply to add weight, and their position in the holes drilled, has in every case been found by trial at the factory.
Moving these screws toward the cut ends of the balance changes the temperature compensation. The nearer a screw is to the cut end of the balance segment the farther in or out it will be carried with changes of temperature and consequently, the greater the effect will be.
A watch with a brass balance and tempered steel hairspring, if regulated in a temperature of 40° F, when run for 24 hours in a temperature of 90° F, will lose about 6 minutes, 15 seconds per day, or about 15 seconds per hour. Of this, 47 seconds is due to the increase of size of the balance, due to the heat, while the rest, 5 minutes, 28 seconds, is due mainly to the loss of elastic force of the hairspring.
In a compensation balance, the screws are adjusted so that these losses will be made up by what amounts to a practical reduction in the mean diameter of the balance. This is all there is to temperature adjustments, and, as stated, they are determined by trial for each balance by running them at 40° and 90° in the factory.
It can be seen, therefore, that it is best to let the balance alone unless one is provided with means of maintaining accurate temperatures over extended periods while a watch is being rated. The watch factories have these necessary facilities, yet they find it necessary to determine by actual trial the proper position of the screws on each balance.
It is, therefore, a good plan to let the screws alone, except to screw them in or out a trifle if the balance is found to be out of poise. Every watchmaker should by all means have a good poising tool and test each balance for poise as this will have an effect not only on the timing but also on the pendant positions in adjusting.
A balance heavier on one segment than on the other will not perform at the same rate in the pendant positions. Screwing the timing screws out will slow the rate; screwing them in will reduce the practical size of the balance by bringing that much weight nearer the center and hence the balance will run faster.
With a well-poised balance properly adjusted for temperature at the factory, and not monkeyed with afterward, it is possible to bring the watch to time, solely by means of the timing screws with the regulator in the center and this is the method followed in the factories.
Many fine watches and pocket and marine chronometers have no regulator. The manufacturer brings the watch to time in the factories by timing screws, and leaves off the regulator deliberately, so that the purchaser cannot change the rate. If the buyer finds the watch is losing or gaining he must then take it to a jeweler who knows what he is doing . . . one who can change the rate with a slight touch to the timing screws.