HEAT. 



The contraction of iron in cooling must be allowed for in iron castings, 

 the pattern always having to be made slightly larger than the casting 

 required. 



In measuring base lines for trigonometrical surveys " compensation 



bars," first designed by 

 General Colby for the 

 Indian Survey, are now 

 used. To understand 

 their construction, sup- 

 pose that we have two 

 parallel bars, AB of brass 

 and CD iron, held at 

 their middle points PQ. 

 Let these be of equal 

 length at some given tem- 



C"- 



D 



A A 



B B 



FIG. 17. Compensation Measuring- Bar. EP, of 

 invariable length ; AB, brass; CD, iron. 



perature. Let their tem- 

 perature rise and let their 

 expansions be as 3:2. 



If then AB expands to A'B' and CD to C'D', AA' : CC' = BB' : DD' = 3:2. 

 If A'C' be produced to cut AC produced in E then EA : EC = 3:2 and E 

 is a fixed point. Similarly F is fixed and the distance EF is indepen- 

 dent of temperature variations so long as the two bars are all at one 

 temperature. It is usual to make AB of brass and CD of iron. ACE, 

 BDF are steel pieces jointed at A and C, B and D, and marks are made 

 at the extremities E and F. 



In a pendulum clock, since the rate depends on the length of the 

 pendulum, the time of swing tends, as already pointed out, to become 

 longer in summer than in winter. An iron pendulum, for example, 

 increases by about '000012 of its length for a rise of 

 1, altering the rate by half this proportion, and so -A ~ 



tending to make the clock lose about half a second 

 per day. With a variation of 20 or 30, the change 

 of rate becomes very serious. This effect of tempera- 

 ture is eliminated as far as possible by employing a 

 " compensating " pendulum, made up of two metals, 

 arranged in such a way that the distance between 

 the centres of suspension and oscillation the effec- 

 tive length remains constant. 



The form most commonly used* is the "grid- 

 iron" pendulum, invented by a clockmaker named ^~^ ^~^ B---*fi 

 Harrison about 1720. If we make an arrangement _ ig Principle of 

 of bars of iron and brass as in Fig. 18, Hi being Gridiron Pendulum 

 iron, bb brass, and suppose the expansions to be in of 

 the ratio 2:3, then the distance between A and B, 

 measured parallel to the bars along a/3, is invari- 

 able. For suppose that each iron bar expands a length t, and that 

 each brass bar expands a length b, then 3i=26. If we first suppose 

 the iron alone to expand, it is evident from the arrangement that, 



* A simple form, on exactly the same principle as that employed by Colby in his 

 compensation bar, was devised by Ellicott and described in the Philosophical 

 Transactions for 1761. 



Iron and Brass 

 Bars - 



