158 Mr Rudge, On the Difference of Potential 



The following is a typical example : 



Spark given by induction coil 5 cm. 



Iron Electrodes. Aluminium Disc. 



Spark length by Micrometer. 



In a similar manner the tube with disc of copper, iron, and 

 silver was compared with the standard and, taking the mean of 

 many experiments, the following figures were deduced : 



Standard Al Cu Fe Ag 



Potential differences] -, _ , OK 10K 10K 



„ , t ,, V 1* 3-0 I'oo rou l - 25 



trom spark lengthj 



The pressure of the air in the tube was about *05 mm. in each 

 case. 



A magnesium disc was next used and this gave a perceptibly 

 higher potential than was the case with aluminium, but owing to 

 the breaking of the tube it could not be directly compared with 

 the series given above. It was however compared with aluminium 

 in another tube. Lead and zinc were also compared with iron, 

 and the following potential ratios obtained : 



Al : Mg = 1 : 12, Fe : Pb = 1 : 075, 

 Fe : Zn = 1 : 1. 



Arranging the metals experimented on in the order in which 

 they (when used as perforated discs near to the cathode) increase 

 the potential of a vacuum tube, the apertures of the discs being of 

 the same size, we got the order : 



Pb Ag (Cu Fe Zn) Al Mg 



1 1-25 1-35 3-5 3-8 



that is, if the introduction of a lead disc increased the potential 

 difference of a certain tube by a definite amount, then an 

 aluminium disc would increase the potential by about 3*5 times the 

 amount. 



It was found that a glass disc produced very much the same 

 effect as Fe. 



