316 Prof. J, J. Thomson on Cathode Rays. 



difference between the cathode and anode depended greatly 

 upon the metal used for the electrode ; the pressure being the 

 same in all cases. 



To test this point further I used a tube like that shown in 

 fig. 6, where a, b, o are cathodes made of different metals, 

 the anodes being in all cases platinum wires. The cathodes 

 were disks of aluminium, iron, lead, tin, copper, mercury, 

 sodium amalgam, and silver chloride ; the potential-difference 



Fig. 6. 



a b c 



between the cathode and anode was measured by Lord Kelvin's 

 vertical voltmeter, and also by measuring the length of spark 

 in air which, when placed in parallel with the anode and 

 cathode, seemed to allow the discharge to go as often through 

 the spark-gap as through the tube. With this arrangement 

 the pressures were the same for all the cathodes. The potential- 

 difference between the anode and cathode and the equivalent 

 spark-length depended greatly upon the nature of the cathode. 

 The extent of the variation in potential may be estimated from 

 the following table : — 



p , -, Mean Potential-Difference 



oatnocie. between Cathode and Anode. 



Aluminium 1800 volts. 



Lead 2.100 „ 



Tin 2400 „ 



Copper 2600 „ 



Iron 2900 „ 



The potential-difference when the cathode was made of 

 sodium amalgam or silver chloride was less even than that of 

 aluminium. 



The order of many of the metals changed about very 

 capriciously, experiments made at intervals of a few minutes 

 frequently giving quite different results. From the abrupt 

 way in which these changes take place I am inclined to think 

 that gas absorbed by the electrode has considerable influence 

 on the passage of the discharge. 



I have much pleasure in thanking Mr. Everitt for the 

 assistance he has given me in the preceding investigation. 



Cambridge, Aug. 7, 1897. 



