Disintegration of the Aluminium Cathode. 351 



In a similar manner, the discharge was sent through a 

 chlorine and an iodine tube in series, the pressures in the 

 tubes being kept practically the same. The disintegration 

 in the iodine tube was much more rapid than in the chlorine 

 tube, and after the discharge had been running for an hour, 

 the deposit in the iodine tube appeared to be three or four 

 times as great as that in the chlorine tube. 



Thus it would seem that the rate of disintegration of the 

 aluminium cathode in the halogen gases increases in the 

 general order of their molecular weights. This is in accord 

 with the investigations of Kohlschiitter* with a platinum 

 cathode in hydrogen, helium, nitrogen, and argon. As 

 J. J. Thomson t has pointed out, the disintegration of the 

 cathode is probably brought about by the bombardment by 

 positive particles, which give to the atoms of the metal 

 sufficient energy to escape ; and that the heavier molecules 

 may more readily receive multiple charges, and that thus 

 their kinetic energy is increased, as shown by the relation, 

 Kinetic Energy = V.e, where V is the cathode fall in poten- 

 tial and e the electric charge on the molecule. 



In the above cases, the discharge was excited by an 

 ordinary induction-coil. Later, a storage battery giving 

 2400 volts was employed. The total voltage between the 

 electrodes was measured by means of a Braun electrometer, 

 and the current strength employed was read on a railliammeter. 

 Tt was found difficult to maintain a discharge from the small 

 cells through the long tubes of fig. 1 type, so a shorter tube 

 .(fig. 3) was employed. The tube was 15 cm. long and 



Fii 



< 



>u 



y 



=r* TO PUMP, ETC. 



2*5 cm. in diameter. The cathode consisted of an aluminium 

 wire 2 mm. in diameter and 3 cm. long. 



It was found that some disintegration occurred at lower 

 voltages, yet the spluttering was not rapid until the voltage 

 passed 1000 volts. In the case of iodine the cathode did not 

 disintegrate rapidly until the voltage had reached some 

 1500 volts. This increase in voltage may be due to a 



* Zeitschr. f. Electroch. xii. p. 365 (1906). 



t ' Rays of Positive Electricity/ p. 10-4 (1913). 



