46 Mr Lyman, Note on a Potassium Amalgam Cathode, etc. 



U-tube was replaced by a cylindrical one, of 22 cm. internal 

 diameter, and with a distance of 9'1 cm. between the anode and 

 cathode. As before, an aluminium plate served as anode. The 

 tube was held vertical with the cathode downward. A D'Arsonval 

 galvanometer served to measure the current, the potential differ- 

 ence was read by a Kelvin voltmeter as before. The tube was 

 filled with air, and mercury was run in at the bottom to form the 

 cathode ; with this arrangement the tube was exhausted step by 

 step, and the potential difference and current were read at each 

 step. The current was kept as nearly constant as possible. After 

 the readings were completed the mercury was removed, the tube 

 was cleaned, and the 1 per cent, potassium amalgam was intro- 

 duced as cathode. Under these conditions a new set of observa- 

 tions were taken. 



From these data a curve for a current of '26 milli-amperes was 

 drawn. 



The results are similar to those obtained in the first experi- 

 ment. I 7 ! does not differ greatly from V 2 down to a pressure of 

 '3 mm., but from here V x increases much faster than V 2 . At a 

 pressure of '08 mm. the ratio of V y to V 2 is 20 to 7. 



Though at first the potassium amalgam cathode was protected 

 from illumination, this precaution proved unnecessary, for a series 

 of experiments showed that exposure of the cathode to diffused 

 light from an external source produced no effect on the result. 



An alloy of sodium and potassium obtained by mixing the 

 metals in the proportion of their combining weights gave results 

 similar in character and magnitude to those obtained with the 

 potassium amalgam. 



A similar set of experiments were also undertaken with a 

 cathode of 1 per cent, sodium amalgam. The general nature of 

 the results was the same as with potassium, but not to such 

 a marked degree. 



The conclusion to be drawn from this investigation is that, 

 with low pressures, the potential difference necessary to send a 

 discharge through air is less when a potassium amalgam is used as 

 cathode than when the cathode is of pure mercury. I am unable 

 to decide whether the result is produced by the action of the light 

 inside the tube upon the amalgam, and is a photo-active effect, or 

 whether the electric force to which the amalgam is subjected is 

 the cause of its action. 



This experiment was carried out in the Cavendish Laboratory, 

 and I wish to express my best thanks to Prof. Thomson for much 

 kindly interest and advice during the progress of the work. 



