250 gray: linear expansivity 



• 



AB are avoided by grinding its ends to form portions of a horizontal 

 cylinder, the axis of which passes thru the center of the bar. The 

 wires should be protected from air drafts. 



In the arrangement of fig. 1 the change in the distance separating the 

 wires is the same as the elongation of AB. When adjustments are 

 properly made, the damping is so effective that the wires appear per- 

 fectly still without the need of any such stable support as a pier; yet 

 if disturbed, even by such vigorous treatment as removing and replacing 

 the oil bottles, they promptly return to their original positions. Re- 

 peated trials have shown that a precision of better than a micron is 

 easily obtainable in measuring the distance between two wires thus 

 suspended. 



In the arrangement of fig. 2 deduction of the length of AB from the 

 measurements is a simple matter. The influence of possible changes 

 in CD can be rendered negligible by suitably controlling its temperature 

 and by placing it far enough away from AB and EF; but if this is not 

 feasible, the proper correction can usually be computed without diffi- 

 culty. 



Since the passages thru which the wires enter the inaccessible 

 region may be very long and of cross-section barely sufficient to permit 

 free motion, the effect of the openings upon conditions within (such as 

 temperature distribution) can be reduced to a minimum. If it is de- 

 sired to work in a vacuum, or in some special atmosphere, the passages 

 may be extended on the outside by an envelope completely incasing 

 the wires and the external bar or damping vanes. The motions can 

 be observed through plate-glass windows. A further decided advan- 

 tage of the method consists in eliminating certain optical difficulties 

 which present themselves when direct vision thru openings into the 

 interior is attempted. 



The production of temperature uniformity. If a region in which 

 no heat is generated or absorbed be surrounded by an isothermal 

 envelope maintained constantly at the same temperature, all 

 points within will ultimately come to this temperature. The 

 approach to thermal equilibrium will be hastened by anything 

 which increases the influence of radiation, conduction, or con- 

 vection within the region. The main difficulty of the problem, 

 then reduces to that of securing such an isothermal envelope. 



Tf the region in question has, by any suitable means, been 

 heated uniformly to the desired temperature T, then any pro- 

 cedure which will eliminate heat loss or gain at every point of 

 the surface will maintain T constant and uniform. The rate 

 dQ/dt at which heat will be conducted away from any portion 

 of the bounding surface in consequence of a normal temperature 



