64 KENNELLY-SANBORN— INFLUENCE OF [April 24. 



The pressure of the air in the tank was controlled by pumps con- 

 nected with the tank. The tank was fairly tight and ordinarily held 

 its pressure steadily during a test. A large glass U-tube containing 

 mercury was connected with the tank. The difference of level be- 

 tween the mercury in the two arms of the U, corrected for tempera- 

 ture, gave the difference of pressure between the air inside and out- 

 side the tank. The absolute pressure of the air in the tank was thus 

 the sum of the U-tube pressure and the corrected barometer pressure 

 outside. This absolute pressure was expressed in " bars " or C.G.S. 

 units (dynes per sq. cm.), by allowing 75.009 cm. of mercury to i 

 megabar or 10'' bars.* 



Hot- Wire Temperatures. 



Two hot resistances were selected for the 32 cm. length of test- 

 wire in different series of tests; namely one at 8.44 ohms, and the 

 other at lo.o ohms, corresponding to temperatures of 410° C. and 

 558° C. respectively, by extrapolation from the calibration test be- 

 tween 0° C. and 100° C. indicated in Fig. 6. These temperatures are 

 therefore inferred by resistance. If the temperatures of the wire 

 actually differed from the above inferred values, the values of linear 

 convection here deduced would be correspondingly changed ; but the 

 comparative results would be unchanged. So far as the main sub- 

 jects of enquiry are concerned, it is sufficient that the wire returns 

 to one and the same definite temperature when heated electrically to 

 one and the same resistance. With the air-temperature in the tank 

 in the neighborhood of 20° C, the inferred temperature-elevation 

 of the test-wire by resistance was 390° C. and 538° C. About ten 

 series of speed-measurements were made at each of these elevations, 

 with different air-pressures. 



The following table gives one series of tests as an example. 



* " Les Recents Progres du Systeme Metrique," Paris, Gauthier-Villars, 

 1907. PP- 30-31. 



