gray: linear expansivity 257 



uniformly heated if lateral loss and supply are uniformly dis- 

 tributed by some such construction as that just outlined in the 

 preceeding paragraph. And in cases where it may not be practi- 

 cable to heat the ends as hot as the interior, much is still to be 

 gained by bringing them as near to the proper temperature as 

 circumstances permit. 



The expansion of cold-drawn Bessemer steel. Some results 

 obtained on the first trial of the methods outlined in this paper 

 are exhibited by Fig. 5, which shows the expansion of a steel 

 bar 30 cm. long. The elongations were measured by sighting 

 on platinum wires 1/30 mm. in diameter suspended over the 

 ends as indicated in Figs. 1 and 4. Changes in the distances 

 separating the axes of the observing microscopes were determined 

 by reference to wires similarly suspended from another bar kept 

 so nearly at a constant temperature that any changes in it could 

 readily be calculated. The unannealed cold-drawn steel was 

 first subjected to several alternations in temperature, extending 

 from that of the room up to between 150° and 380? In con- 

 formity with the experience of other observers, the bar was 

 always found noticeably shorter upon cooling than when at the 

 same temperature before the preceding heating; and it was found 

 to return slowly towards its original length if left undisturbed 

 at room temperature. The bar was then heated to about 800° 

 and annealed by slow cooling over night. A few days later the 

 measurements represented by the lower curve in Fig. 5 were made 

 in the order indicated by the numbers, the curve passing almost 

 exactly through all the points obtained. After allowing the 

 bar to rest undisturbed for about a month, a new series of similar 

 measurements was made with the upper curve as the result. 

 The lengthening accompanying the period of rest seems to have 

 extended throughout the whole temperature range — nearly 700°C. 

 A suggested application to gas thermometry. Thermal expan- 

 sion of the material forming the thermometer bulb necessitates 

 corrections for changes in volume, which become of increasing 

 importance as the temperature range is extended. The diffi- 

 culties involved in determining this expansion have been recog- 

 nized by investigators in the field of precision of gas thermometry. 



