Horn & Whistle Magazine: Source for Horns, Whistles, Sirens, Collecting signal devices and Related Information

    Temperature compensation in a watch is derived through an ingenious method which changes the radius of gyration of the balance wheel. In all railroad grade watches, and indeed all better grade watches, the balance wheel is not a complete wheel, but consists of two curved arms attached at one end to two spokes, and the other ends are free. The curved arms appear at first glance to look like a continuous circle, but inspection with a microscope reveals that they are attached to their spokes at only one end. Each arm is also a bimetallic strip.
    Bimetallic elements consist of two metals having differing coefficients of expansion bonded together. If the temperature increases, one metal expands more than the other. This action makes the strip bend in the direction of the metal having the lower expansion coefficient. Likewise, a decrease in temperature will make the metal bend away from the metal which has the lower coefficient. As we consider this action and the above description of a watch balance wheel, we can easily see how this action will change the effective average diameter of the watch balance wheel with a change in temperature, thus changing its radius of gyration and thereby its frequency of oscillation.

    The motion is imparted to the balance by the fork of the lever pushing the roller jewel. Railroad watch requirements specify that a so-called guard pin on the lever has to pass through a small cut-out on the other roller, referred to as the safety roller.

Figure 6. Illustration of escape wheel and lever showing how jewelled pallet stones contact the teeth of the escape wheel making the lever rock back and forth on its shaft or arbor. The other end of the lever imparts reciprocating rotary motion to the balance wheel. In a RR watch, the escape wheel makes one revolution every six seconds. In lower grade wark, steel pins replace the pallet stones.

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    The oscillating or back-and-forth motion of the balance wheel results from the rocking action of the lever, the other end of which looks like a fork, figure eight, right. This acts upon the small jeweled pin on the roller assembly, figure seven, right. The roller assembly mounts on the shaft of the balance wheel, called in watch work the balance staff.