The position rating as well as the final regulation is generally done in normal temperature which produces a rate from two to four seconds faster than the heat extreme and it is to be expected that the pocket rate will be slower because the temperature will be higher than normal. This loss may not be the full amount of the middle error as it would depend upon the actual temperature encountered for the entire twenty-four hours and the watch may only be subjected to the pocket temperature for a part of this period. This works in exactly the same way in a lower temperature, as the variation is a loss in either direction from the middle or normal temperature and in case that the watch should be subjected to a freezing temperature at night the result will be a loss during that period.
As an example we will assume the regulation of a watch in which the temperature rate at the extremes of 40° and 90° Fahr. is perfect, while at the temperature of 70° it will time four seconds fast.
Now if this watch is regulated to no variation in the normal temperature it will be plainly seen that there will be a loss of four seconds per day if the watch is placed in service at either of the temperature extremes. If it had been regulated to run four seconds fast in the middle or normal temperature it would time more nearly correct in the pocket.
It is safe to assume that the watch will lose its proportional rate with a lesser change in temperature and for this reason it is of advantage to finally regulate all watches from two to four seconds fast in the rack rather than to time them just correct.
[78.]Some Practical Reasons for Slow Rates.
There are additional reasons for the suggestion of timing watches a few seconds fast rather than just correct. Among them may be mentioned the fact that many watches are carried in the left vest pocket, and that in this instance they very often assume the pendant right position which is generally a trifle slow compared to pendant up in most watches of close adjustment. Magnetism to any extent whatever always causes a slow rate and this will have its effect whenever the balance, hairspring, regulator, regulator spring or pallet are slightly effected or when the mainspring, large winding wheels or case springs are considerably charged and experiments have shown that in no instance has a fast rate been produced from this cause.
The gradual weakening or loss of elastic force of the hairspring is also a factor to be considered.
There are some influences which cause a gaining rate that to some extent may offset these losses, although in the absence of necessity for cleaning or other repairs these influences are slight in comparison to the natural and possible causes for a slow rate.