In selecting and fitting a spring that will be one coil larger or smaller, it should be noted that the inner coil of the original spring and that of the new spring are approximately the same distance from the collet. For if there was considerable space between the collet and inner coil of the original spring, and the new spring was colleted quite close, there might be the addition of an extra coil in the inside only. This was found to produce only a very slight correction, compared to that obtained by the addition of a complete outer coil. These tests indicate that the proportion of strength of the spring in the temperatures varies with any appreciable change in length while slight changes make practically no difference.
20. _Example Demonstrating that Temperature Variation is not Always Due to the Balance and Spring._
+--------------------------------------------------+ | No. .................... Make................... | +--------+-----+-----+-----+-----+-----+-----+-----+ | HEAT | -10 | -10 | + 4 | + 4 | + 1 | + 1 | | +--------+-----+-----+-----+-----+-----+-----+-----+ | NORMAL | - 6 | + 4 | + 5 | + 1 | + 4 | + 3 | | +--------+-----+-----+-----+-----+-----+-----+-----+ | COLD | +12 | +18 | + 1 | - 4 | + 7 | + 3 | | +--------+-----+-----+-----+-----+-----+-----+-----+ 28 8 2
The following example is submitted to show that temperature variation is not always due to the balance and spring, and that the general condition of the watch may be responsible. The second column of Fig. 7, indicates an error of twenty-eight seconds slow in heat with all screws assembled in the holes nearest the free ends of the rims.
Examination proved that the motion of the balance in cold was reduced to about one-fourth of a turn. In heat the arc of motion was at least one full turn. This difference in motion was sufficient to prove that there was some binding in the train.
A very close fitting of the escape pivots was found and this undoubtedly caused binding of the pivots in heat due to slight expansion. Expansion of the stone would also tend to close the hole, and while the degree of temperature would hardly have any bearing on this point it is sufficient to show in what direction the tendency would be. The fourth wheel end shake was very close and probably caused binding of the wheel in cold, due to greater contraction of the bridge than of the fourth pinion. Furthermore the mainspring was only 0.02 of a millimeter narrower than the space in the barrel box. This no doubt also caused binding through greater contraction of the barrel than occurred in the mainspring.