Mr Lyman, Note on a Potassium Amalgam Cathode, etc. 4o 



Note on the behaviour of a Potassium Amalgam Cathode in 

 a Vacuum Tube. By Theodore Lyman, Jefferson Physical 

 Laboratory. 



[Read 24 November 1902.] 



The photo-electric behaviour of sodium or potassium amalgam 

 and of the sodium-potassium alloy is well known, and has been 

 investigated by Elster and Geitel, among others. The present 

 note refers to the action of these same amalgams when used as 

 cathodes in an exhausted discharge tube. 



The first experiment was made with an amalgam containing 

 one per cent, of potassium in mercury. The tube had the form of 

 an inverted U. One of the lower arms contained about a centi- 

 metre of pure mercury, the other the same quantity of the 

 amalgam under investigation. By means of suitable connecting- 

 wires sealed into the tube, either the mercury or amalgam could 

 be made the cathode. An aluminium plate 8 mm. in diameter was 

 sealed into the bend of the U tube, and served as a permanent 

 anode. The internal diameter of the tube was 1*2 cm., the 

 distance of either cathode to the common anode was 15 cm. The 

 current was supplied by a Wimshurst machine, driven by a 

 small electric motor. The potential difference between anode 

 and cathode was measured by a Kelvin electrostatic voltmeter. 

 The limb of the tube containing the potassium amalgam was 

 covered by several thicknesses of black paper to protect it from 

 external illumination. The pressure in the tube was measured by 

 a McLeod gauge. 



The tube was filled with dry air and exhausted step by step. 

 At each step the pressure was read. The difference of potential 

 between the terminals of the tube was also taken, first with the 

 mercury as cathode, second with the amalgam as cathode. The 

 aluminium plate at the bend of the tube remained the anode 

 throughout the experiment. 



If V 1 be the potential difference with the mercury cathode and 

 V 2 that with the amalgam cathode, then the observed facts are as 

 follows : — At a pressure of about 1 mm. of mercury, V 1 was equal 

 to V 2 ; as the pressure decreased V ± remained nearly equal to V 2 

 until the region of - 2 to '3 mm. pressure was reached, here V x 

 began to increase much faster than V 2 . At a pressure of "08 mm. 

 V 1 was twice as large as V 2 , and at a pressure of # 06 mm. the ratio 

 was 3 to 1. The experiment was repeated many times. 



In order to study the phenomenon under conditions of constant 

 current, the Wimshurst was replaced by a number of small storage 

 cells, connected to the tube through a suitable resistance. The 



