A watch balance is made with a rim of brass encircling and firmly united to the rim of steel. In order to permit heat to have the desired effect upon this balance, the rim is completely severed at points near each of the arms of the wheel. If we apply heat to this balance the greater expansion of the brass portion of its rim would cause the free ends to curl inward.

In order to obtain exactly 18,000 vibrations of the balance in an hour, it will be seen that the weight of the wheel and the strength of the hair spring must be perfectly adapted each to the other. The shorter the spring is made the more rigid it becomes, and so the regulator is made a part of the watch, but its action must be very limited or its effect on the spring will introduce other serious disturbances. The practical method of securing the proper and ready adaptation of balances to springs is to place in the rims of the balance a number of small screws having relatively heavy heads. Suppose now that we have a balance fitted with screws of the number and weight to exactly adapt it to a spring, so that at a normal temperature of, say, 70 degrees, it would vibrate exactly 18,000 times per hour. When we place the watch in an oven the heat of which is 95 degrees, we might find that it had lost seven seconds. That would show that the wheel was too large when at 95 degrees, although just right at 70 degrees. Really, that is a very serious matter - it would lose at the rate of 2 4/5 minutes in a day. But after all it need not be so very serious, because if we change the location of one screw on each half of the balance so as to place it nearer the free end of the rim when the heat curls the rim inward, it will carry a larger proportion of the weight than if the screws had not been moved. It may require repeated trials to determine the required position of the rim screws, and both skill and good judgment are essential. It will be readily understood that numerous manipulations of this kind constitute no small items in the cost of producing high-grade watches.

Large quantities of the cheaper class of watches are now made by machinery in the United States, Switzerland, France, Germany and England. They are generally produced on the interchangeable system, that is, if any part of a watch has become unfit for service, it can be cheaply replaced by an exact duplicate, the labor of the watch repairer thus becoming easy and expeditious.