Nov. 2, 1876] 



NATURE 



PRINCIPLES OF 



TIME-MEASURING 

 PATHS'^ 



IV. 



APPA- 



Balance Springs. 



THE earliest watches were constructed, so far as the 

 escapement and balance were concerned, upon ex- 

 actly the same plan as the clock from Dover Castle, and 

 in this condition they must have perpetually remained 

 (useless as time-measurers), but for the invention of the 

 balance-spring (sometimes called the pendulum-spring, 

 on account of the uniformity it imparts) by Dr. Hooke. 



This spring bears the same relation to the balance 

 which gravity does to the pendulum. Everybody, I 

 presume, knows the form it usually takes in watches ; 

 and in chronometers it is coiled up around an imagi- 

 nary cylinder. This spring (as in the case of the pen- 

 dulum) absorbs the energy of the impulse, and when 

 the balance has reached the limit of its swing, redelivers 

 to it all it has received. A watch is regulated by shorten- 

 ing or lengthening the balance-spring, which makes it 

 more or less rigid. 



Watches and chronometers vary their time to a 

 much greater extent upon any change of temperature 

 than clocks do. For instance, if we regulate a chro- 

 nometer without any compensating arrangement, with 



Fig. 19. 



a balance-spring of steel, to go right at a temperature 

 of 32 deg., when we raise the temperature to 100 deg. it will 

 lose 6 minutes 25 seconds a day, whereas a clock with an 

 ordinary steel rod pendulum would barely lose 20 seconds 

 for the same change. This great difference is owing to 

 the alteration in elasticity of the balance-spring (the effect 

 really being the same as if, with reference to the pendu- 

 lum, we could reduce the* force of gravity). 



Different materials are in this respect differently 

 affected. Whereas a chronometer with a spring of steel 

 will lose 6 minutes 25 seconds a day for 68 deg, rise in 

 temperature, one with a gold spring would lose 8 minutes 

 4 seconds ; a palladium spring would lose 2 minutes 

 31 seconds ; a glass ^ spring would lose 40 seconds. On 

 account of the large amount of compensation required, 

 quite a different plan has to be employed to that made 

 use of in clocks. 



Suppose I take two thin strips of brass and steel, and 

 fasten them rigidly together (the best plan is to pour the 

 melted brass upon the steel), what will happen when there 

 is any change of temperature ? Imagine the temperature 

 to rise, both expand, but the brass more than the steel, 

 and how it manages this, being rigidly fastened to it, is 



* I<ectures by Mr. H. Dent Gardner, at the Loan Collection, South 

 Kensington. Concluded from vol. xiv. p. 575. 

 ^ The glass spring was the invention of the late Fredk. Dent. 



by bending round the steel into a curve, of which ii (the 

 brass) is upon the outside. 



Fig. 19 represents the first form of compensation applied 

 to watches, C A is our compound bar (the steel, shaded 

 black, being nearest the spiral), the extremity of which 

 carries two pins applied to the balance-spring as the ordi- 

 nary regulator. 



When the temperature rises the brass expands and 

 bends round the steel, shortening the balance-spring, and 

 thus compensating for its loss of elasticity due to the rise 

 in temperature. The reverse action takes place when the 

 temperature falls. 



The plan adopted now-a-days is the compensation- 

 balance (see Fig. 20), The rims, RiR2, are formed of two 

 strips of brass and steel, the brass being upon the out- 

 side. When the temperature rises, the brass expands 

 more, and bends in the rims, carrying the weights w 

 tOiV^ards the axis of motion a sufficient distance to com- 

 pensate for the loss of time due to the loss of the spring's 

 elasticity. 



As you see, the action of the compensation may be 

 readily increased by shifting the weights, nearer to the 

 ends of the rims. 



Where a chronometer is exposed to very wide ranges of 

 temperature, there is another error (called the secondary 



Fig. 20. 



error) introduced. If, for example, we take a chronometer 

 with such a balance as just described, and so adjust the 

 compensation- weights that it shall go right at atemperature 

 of 66 deg, and 32 deg., we shall find that when we expose 

 the chronometer to a temperature of 100 deg. it will lose 

 about four seconds a day ; and we cannot correct it, for 

 if we advance the compensation-weights along the rims 

 to increase their action in the heat, we shall also increase 

 it in the cold, and then the chronometer will lose in that 

 direction. The best we could do would be so to adjust 

 the weights as to make the chronometer lose two seconds 

 a day in the heat, and two seconds a day in the cold. 



The cause of this error is that the time of the swing of 

 a balance depends not directly upon the distance of its 

 weights from the axis of motion, but upon the square of 

 that distance ; and it therefore requires a greater amount 

 of motion inwards to produce the same effect as any given 

 motion outwards. 



The following is one of the plans adopted for the cor- 

 rection of this error (see Fig, 21) : — 



F B is a flat bar composed of brass and steel fastened 

 together, the brass being beneath, ll are two loops 

 also formed of brass and steel, the brass being inside. 

 The compensation weights, \v w, are mounted upon 



