﻿808 Mr. K. K. Smith on Negative 



300° hotter than usual before the current was large enough 

 to be read on the microammeter. At this higher temperature, 

 1890° K., the current was now only 0*014 milliamp. per 

 sq. cm., as compared to 5*36 milliamp. per sq. cm. at the 

 same temperature in series 2. In other words, the thermionic 

 current had been reduced to l/380th part of what it had 

 been. It began to increase very slowly, although the tempe- 

 rature was constant. The other points of series 5 were 

 determined at once, without waiting to see if the current 

 would become steady at the lower temperature. The current 

 increased with the temperature much more rapidly than 

 before, and approached the preceding series so that at the 

 highest temperature it was 60 per cent, of the current at 

 the same temperature in series 4. About 2500° K. a blue 

 glow appeared after continued heating, and the copper anode 

 became red-hot. 



The above experiments were performed before Dr. Lang- 

 muir's paper * on thermionic currents was published. They 

 confirm his conclusion that the effect of residual gases is 

 to decrease the thermionic current, especially at low tempe- 

 ratures. The fact that considerably larger currents, at the 

 same temperatures, have been obtained by the writer is 

 undoubtedly owing to better vacuum conditions. According 

 to the above paper, the normal vacuum curve was obtained 

 at a pressure of 0*07 micron, and gives the following values : 

 a = 34xl0 6 amps, per sq. cm., 6 = 55,500. By the use of 

 charcoal and liquid air, as described before, it has been 

 possible to keep the pressure as low as 0*001 micron, or 

 less. 



In Lamp 1, so long as the thermionic current did not 

 exceed those employed before the lamp was sealed, the 

 vacuum was practically perfect, and very large currents 

 were obtained. At 2000° K. 26 milliamperes per sq. cm. 

 were measured. Later, the excessive heating and bombard- 

 ment of the anode by electrons liberated occluded gas, which 

 reduced the thermionic current. The presence of gas is 

 proved by the blue glow which appeared. There is no 

 reason to suppose that any appreciable amount of gas was 

 present previous to the excessive heating. Tests have been 

 made at various times with an induction-coil, on similar 

 lamps which had not been overheated, and no indications of 

 gas were ever found. The potential difference and the size 

 and shape of the anode were such as to permit the normal 



* Pliys. Eev. ii. p. 450 (1913). 



