490 Mr. Gr. L. Addenbrooke : A Study of Franklins 



important action which bears largely on a class of problems 

 which has much practical importance. 



This is known as the "Franklin Experiment" on the 

 Leyden Jar with movable coatings. 



The experiment in its simplest form is briefly thus :— The 

 jar is charged, the inner coating is lifted out by an insulated 

 hook, and this inner coating is then touched against the 

 outer coating and put back. It is found that when taken 

 out the inner coating carries scarcely any charge, and that 

 when it is put back a full or nearly full discharge can be 

 got from the reconstituted jar. 



The experiment is sometimes varied by placing the charged 

 jar on an insulating stand and taking away by insulated 

 tongs both the covers, leaving the glass jar exposed. Both 

 covers are found to be practically uncharged, they are then 

 touched against each other. The whole jar and coatings are 

 next reconstituted, when a full discharge can be obtained. ■ 



This has been taken to mean that when a dielectric 

 between two electrodes is charged, if the electrodes are 

 taken away while still charged, the energy of the charge is 

 left stored in the dielectric, and remains there until the 

 electrodes are restored and short-circuited, or the charge dies 

 down by some indefinite form of leakage. 



For a long time I could not reconcile this experiment and 

 its general interpretation with the views I had been led 

 io form of electrical actions in dielectrics in other respects. 

 I therefore searched through a number of works for the pur- 

 pose of finding any variations there might be in the methods 

 of performing the experiment, or in the explanation of it, 

 when I noticed that there seemed to be no mention of it 

 in Faraday's or Maxwell's work. It is not mentioned in 

 Gordon's ' Electricity and Magnetism,' nor in Sir J. J. 

 Thomson's Text-book, though it is described in Poynting and 

 Thomson's ' Electricity and Magnetism ' of 1914. This was 

 significant, though it seemed almost equally significant that 

 if the experiment was omitted for any reason, no reasons 

 should be given for so doing. 



At last, to clear up the matter there seemed no other 

 course but to investigate it further oneself. 



In considering how to do this the first point which struck 

 me was that a glass jar was invariably mentioned as the 

 dielectric. 



The question arose what would happen if another dielectric 

 were substituted. 



Finally, solid paraffin was selected as a suitable substitute 

 on account of its high insulating properties, and because its 



