gray: temperature uniformity 135 



in thermoelements of the best quaUty, and probably less than 

 the uncertainties at present existing in our knowledge of the 

 temperature scale itself. The method has been applied to a 

 furnace designed to heat uniformly bars under measurement for 

 thermal expansivity; but the application of it clearly is not lim- 

 ited to problems of this general character. 



The iron tube of the trial furnace,- together with its contents, 

 is retained without essential alteration. The principal change 

 consists in removing the heating coil from the tube upon which 

 it had formerly been wound and substituting two independent 

 concentric heaters to supply heat uniformly for the entire length 

 of the interior. In the experiments so far made air has formed 

 the insulation, filling the small annular space separating the orig- 

 inal tube from the first heater, and also the larger space between 

 the heaters, except for some layers of mica and of asbestos cloth, 

 in all about 4 mm. thick, wrapped tightly around the ribbon of 

 the inner heater. The outer one is covered in the same way, 

 and the space between this and the casing of the whole furnace 

 is filled with asbestos-magnesia compound. The end plugs con- 

 sisting of two thick blocks of a good heat conductor separated 

 by a thick layer of a poor conductor are attached to large insu- 

 lating heads filling the entire cross-section of the casing. In most 

 of the experiments heat was supplied to the plugs by means of 

 coils filling the grooves turned in the outer conducting blocks, 

 just as in the former furnace; but recently considerable improve- 

 ment was effected by cutting these coils out of action, and sub- 

 stituting flat coils of the same diameter as the outer heater, 

 against the ends of which they bear. 



The concentric heaters are of nichrome ribbon wound longitu- 

 dinally, instead of helically, upon iron pipes. A flat mat is made 

 of the resistor ribbon and strips of micabeston^ woven together 

 in basket fashion. This is wrapped about the heater tube 

 (previously covered with a layer of micabeston), and the oppos- 

 ing ends of each insulating strip are lapped for a short distance 



2 For an illustrated description see the communication cited above. 

 ^ An insulating preparation of mica flakes and a resinous cement pressed into 

 large, thin, flexible sheets. 



