Spectra of Helium, fyc* in the Ultra- Violet. 291 



air-trap, then a discharge-tube for examining the spectrum 

 of the gas, then a mercury cut-off valve, then a second liquid 

 air-trap containing charcoal. There was next another 

 mercur) cut-off, beyond which was a bulb with electrodes 

 for the purification of the gas by sparking with oxygen 

 over P 2 5 , then two tubes, one containing cleveite and the 

 other potassium permanganate, with suitable stopcock. This 

 part of the apparatus was also provided with a palladium 

 tube for the introduction of hydrogen and with a McLeod 

 gauge reading to '0001 mm., and was finally connected with 

 a Graede mercury-pump. The helium was prepared by 

 heating the cleveite, then mixed with oxygen obtained by 

 heating the permanganate, then sparked over the pentoxide 

 for about one hour, then passed into the tube containing 

 charcoal in liquid air, and finally admitted into the apparatus 

 proper. The gas thus obtained was entirely free from 

 hydrogen and from all other impurities that could be de- 

 tected spectroscopically or otherwise. When hydrogen was 

 used it was introduced by heating the palladium tube with a 

 bunsen burner. 



The electrical connexions (excluding those to the coils for 

 producing the magnetic field) used in the majority of the 

 experiments are shown in fig. 2. Aj is a milliammeter or some- 

 times a unipivot microammeter with or without a shunt as 



Fig. 2. 



4-00 V. -h 



/H'l'M'M' |»|i|i|> 



V^VAA/WVWWWWVVV-' 



50 V. + 



-_ V.V\AA i J 





yrWlMlli--- 



S 800 V. 



hl« 



may be required. G is a galvanometer sensitive to 10 -9 

 ampere or in some cases a suspended-coil microammeter. 

 E is a Kelvin electrostatic voltmeter reading from 200 to 

 1000 volts. For lower voltages a Weston voltmeter of suit- 

 able range was used in place of the electrostatic instrument. 



