nutting: ftelttm \s li«;mt standard 221 



PHYSICS.— Helium tubes as light standards. P. G. \Yi tin 

 Bureau of Standards. Communicated by S. W. Stratton. 



The investigation of the properties of electrically conducting 

 gases with a view to their possible use as primary standards of 

 luminous intensity has been in progress at the Bureau of Stand- 

 ards for the past six years. A summary of the results obtained 

 is given below, this includes a report of a recent reproducibility 

 test here given for the first time. 



Of the available gases and vapors, helium is the only one emit- 

 ting light of a suitable color and having reasonable lasting qual- 

 ities. The light is of a yellowish white approximately that of 

 the Hefner flame and of an old type carbon filament glow lamp. 

 The useful life of a helium tube is from 5 to 100 hours depending 

 on its construction and operation. The next best gases would 

 be carbon dioxide and sulphur, but these emit a snow white light 

 and disappear rapidly in carrying a current. 



The form of tube found most suitable has a capillary portion 

 of 2-mm. bore, 2-mm. thickness of wall and about 7 cm. long. 

 The terminal bulbs are 35 mm. in diameter and spherical while 

 the electrodes are aluminum disks 1 mm. thick and 25 mm. in 

 diameter. Recent results indicate that possibly a slightly in- 

 creased bulb diameter might give longer life to the tubes. 



The relation between current and light emitted is very nearly 

 linear over a wide range of intermediate currents, 10 to 35 m.a. 

 Twenty-five milliamperes was chosen as the most suitable stand- 

 ard current. 



The ratio of light emitted to internal energy, current times 

 potential gradient, is not quite constant. The minimum watts 

 per candle for a tube of 2-mm. base, namely, 3.8 w./c.p., occurs 

 at a current between 25 and 30 m.a. Owing to the large and 

 uncertain variations in the potential gradient and the difficulties 

 in its precise determination, attention was directed toward ex- 

 pressing light in terms of current and bore rather than in terms 

 of internal energy. 



The light emitted by a helium tube as a function of current 

 was later determined line by line for each of the six prominenl 

 spectrum lines. These all behave nearly alike, increasing with 



