gray: linear expansivity 



255 



the smallness of the passages required for the suspended wires certainly 

 prevented much disturbance, a very important feature of the whole 

 construction was undoubtedly the type of plug used for closing the ends. 

 This was essentially the same as the simple one described above, with 

 the addition of an independent electrical heating coil wound in a groove 

 within the outside conducting block. This coil made it easy to main- 

 tain the end at a temperature almost the same as that of the interior, 

 thus reducing the longitudinal temperature gradient and thereby the 

 outward conduction of heat. Suitably arranged differential thermo- 

 elements in the plugs, together with a convenient grouping of rheostats, 

 make the temperature control an easy matter. 



30 



i_ 



20 

 1 



20 



1 



Wa ■ . • • - 



I 



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Fig. 4. Electrically-heated furnace for expansion measurements by method of 

 suspended wires, showing double plugs with outside ends independently heated. 



The furnace was built to accommodate bars 30 cm. long. Examina- 

 tion of the temperature distribution throughout this length for various 

 temperatures up to 684°C. showed that, if the outside end of the plugs 

 was within 10° of the interior, no variation exceeding a few tenths of a 

 degree occurred in the right-hand 15 cm. except at the highest tempera- 

 ture to which the furnace was forced, when the extreme deviation was 

 found to be 4°. Since the constantan ribbon burned out in the vicinity 

 of the hottest place the next day, considerably before this temperature 

 was again reached, it seems likely that this deviation was due' to exces. 



