the known results of previous experience with similar balances;
and many watches are sold with compensation balances which have
never been tried or adjusted, and sometimes with a mere sham
compensation balance, not even cut through.
Secondary Compensation.—When chronometers had been brought to great perfection it was perceived that there was a residuary error, which was due to changes of temperature, but which no adjustment of the compensation would correct. The cause of the secondary error is that as the temperature rises the elasticity of the spring decreases, and therefore its accelerating force upon the balance wheel diminishes. Hence the watch tends to go slower.
In order to compensate this the split rim of the balance-wheel is made with the more expansible metal on the outside, and therefore tends to curl inwards with increase of temperature, thus diminishing the moment of inertia of the wheel. Now the rate of error caused by the increase of temperature of the spring varies approximately with the temperature according to a certain law, but the rate of correction due to the diminution of the moment of inertia caused by the change of form of the rim of the wheel does not alter proportionally, but according to a more complex law of its own, varying more rapidly with cold than with heat, so that if the rate of the chronometer is correct, say, at 30° F. and also at 90° F., it will gain at all intermediate temperatures, the spring being thus under-corrected for high temperatures and over-corrected for low. Attempts have been made by alterations of shape of the balance-wheel to harmonize the progress of the error with the progress of the correction, but not with very conspicuous success.
We shall give a short description of the principal classes of inventions for this purpose. The first disclosed was that of J. S. Eiffe (sometimes attributed to Robert Molyneux), which was communicated to the astronomer-royal in 1835. In one of several methods proposed by him a compensation curb was used; and though, for the reasons given before, this will not answer for the primary compensation, it may for the secondary, where the motion required is very much smaller. In another the primary compensation bar, or a screw in it, was made to reach a spring set within it with a small weight attached at some mean temperature, and, as it bent farther in, it carried this secondary compensation weight along with it. The obvious objection to this is that it is discontinuous; but the whole motion is so small, not more than the thickness of a piece of paper, that this and other compensations on the same principle appear to have been on some occasions quite successful.
Another large class of balances, all more or less alike, may be
represented by E. J. Dent's, which came next in order of time.
He described several forms of his invention; the following description
applies to the one he thought the best. In fig. 9 the flat
Fig. 9.
crossbar rr is itself a compensation bar which bends
upwards under increased heat; so that, if
the weights v, v were merely set upon upright
stems rising from the ends of the crossbar,
they would approach the axis when that
bar bends upwards. But, instead of the
stems rising from the crossbar, they rise from
the two secondary compensation pieces slu, in
the form of staples, which are set on the
crossbar; and, as these secondary pieces
themselves also bend upwards, they make
the weights approach the axis more rapidly
as the heat increases; and by a proper
adjustment of the height of the weights on
the stems the moment of inertia of the balance can be made to
vary in the proper ratio to the variation of the intensity of the
spring. The cylindrical spring stands above the crossbar and
between the staples.
Fig. 10 represents E. T. Loseby's mercurial compensation balance.
Besides the weights D, D, set near the end of the primary compensation
bars B, B, there are small bent tubes FE, FE with mercury
in them, like a thermometer, the bulbs being at F, F. As the heat
increases, not only do the primary weights D, D and the bulbs F, F
Fig. 10.
approach the centre of the balance,
but some of the mercury is driven
along the tube, thus carrying some
more of the weight towards the
centre, at a ratio increasing more
rapidly than the temperature. The
tubes are sealed at the thin end,
with a little air included. The
action is here equally continuous
with Dent's, and the adjustments
for primary and secondary compensation
are apparently more independent
of each other; and this
modification of Le Roy's use of
mercury for compensated balances
(which does not appear to have
answered) is certainly very elegant and ingenious. Nevertheless an
analysis of the Greenwich lists for seven years of Loseby's trials
proved that the advantage of this method over the others was more
theoretical than practical; Dent's compensation was the most successful
of all in three years out of the seven, and Loseby's in only one.
Loseby's method has never been adopted by any other chronometer maker,
whereas the principles both of Eiffe's and of Dent's methods
have been adopted by several other makers.
A few chronometers have been made with glass balance-springs, which have the advantage of requiring very little primary and no secondary compensation, on account of the very small variation in their elasticity, compared with springs of steel or any other metal.
One of the most important and interesting attempts to correct the temperature errors of a hair-spring by a series of corresponding temperature changes in the moment of inertia of the balance-wheel has been made by means of the use of the nickel-steel compound called invar, which, on account of its very small coefficient of expansion, has been of great use for pendulum rods. In a memoir published in 1904; at Geneva, Dr Charles Guillaume, the inventor of invar, shows that in order to get a true secondary compensation what is wanted is a material having the property of causing the curve of the rim of the wheel to change at an increasing rate as compared with changes in the temperature. This is found in those specimens of invar in which the second coefficient of expansion is negative, i.e. whichare less dilatable at higher temperatures than at lower ones. It is satisfactory to add that such balance-wheels have been tried successfully on chronometers, and notably in a deck watch by Paul Ditisheim of Neuchâtel, who has made a chronometer with a tourbillon escapement and an invar balance-wheel, which holds the highest record ever obtained by a watch of its class.
It is obvious that in order that a watch may keep good time the centre of gravity of the balance-wheel and hair-spring must be exactly in the axis; for if this were not the case, then the wheel would act partly like a pendulum, so that the time would vary according as the watch was placed in different positions. It is exceedingly difficult to adjust a watch so that these "position-errors" are eliminated. Accordingly it has been proposed to neutralize their effect by mounting the balance-wheel and hairspring upon a revolving carriage which shall slowly rotate, so that in succession every possible position of the balance-wheel and spring is assumed, and thus errors are averaged and mutually destroy one another. This is called the tourbillon escapement. There are several forms of it, and watches fitted with it often keep excellent time.
Stop watches or chronographs are of several kinds. In the usual and simplest form there is a centre seconds hand which normally remains at rest, but which, when the winding handle is pressed in, is linked on to the train of the watch and begins to count seconds, usually by fifths. A second pressure arrests its path, enabling the time to be taken since the start. A third pressure almost instantaneously brings the seconds hand back to zero, this result being effected by means of a heart-shaped cam which, when a lever presses on it instantaneously, flies round to zero position. The number of complete revolutions of the seconds hand, i.e. minutes, is recorded on a separate dial.
Calendar work on watches is, of course, fatal to great accuracy of time-keeping, and is very complicated. A watch is made to record days of the week and month, and to take account of leap years usually by the aid of star-wheels with suitable pauls and stops. The type of this mechanism is to be found in the calendar motion of an ordinary grandfather's clock.
Watches have also been made containing small musical boxes and arranged with performing figures on the dials. Repeaters are striking watches which can be made at will to strike the hours and either the quarters or the minutes, by pressing a handle which winds up a striking mechanism. They were much in vogue as a means of discovering the time in the dark before the invention of lucifer matches, when to obtain a light by means of flint and steel was a troublesome affair.
From what has been said it will be seen that for many years the form of escapements and balance-wheels has not greatly altered. The great improvements which modern science has been able to effect in watches are chiefly in the use of new metals and in the employment of machinery, which, though they have altered the form but little, have effected an enormous revolution in the price. The cases of modern watches are made sometimes of steel, artificially blackened, sometimes of compounds of aluminium and copper, known as aluminium gold. Silver is at present being less employed than formerly. The hair-springs are often of palladium in order to render the watch non-magnetizable. An ordinary watch, if the wearer goes near a dynamo, will probably become magnetized and quite useless for time-keeping. One of the simplest cures for this accident is to twirl it rapidly round while retreating from the dynamo and to continue the motion till at a considerable distance. The use of invar has been already noticed.
It would be impossible to enumerate, still more to describe, the vast number of modern machines that have been invented for making watches. It may be said briefly that every part, including the toothed wheels, is stamped out of metal. The stamped pieces are then finished by cutters and with milling machinery. Each machine as a rule only does one operation, so that a factory will contain many hundreds of different sorts of machines. The modern watchmaker therefore is not so much of a craftsman as an engineer. The effect of making all the parts of a watch by machinery is that each is interchangeable, so that one part will fit any watch. It is
