SECTION A.— MATHEMATICAL AND PHYSICAL SCIENCES. 



THE VOLTA EFFECT. 



ADDRESS BY 



PROF. ALFRED W. PORTER, D.Sc, F.R.S., 



PRESIDENT OF THE SECTION. 



Since the last annual Meeting the Association has lost one who on more 

 than one occasion took part in the discussions of Section A. We had 

 hoped that he would be present also at this meeting. I refer to Hendrik 

 Antoon Lorentz, who passed away on February 4, 1928, in his seventy-fifth 

 year. Lorentz had long been regarded as one upon whom the mantle of 

 Clerk-Maxwell had fallen. For his character as a scientist and as a man 

 I may make reference to the columns of ' Nature ' for February 25, 1928. 

 From the group of appreciations there recorded I select the following 

 quotations : ' For many years Lorentz naturally and by general consent 

 took the leading place in every European conference of physicists.' ' His 

 name recalls especially the Lorentz transformation, the culminating point 

 of one phase of electrodynamical theory and the foundation stone of the 

 next.' ' To British investigators Lorentz was ever a most sympathetic 

 figure. This was due partly to his mastery of our language, partly to 

 his keen admiration of the work of the great English leaders of his time, 

 and above all to the transparent kindliness and charm of his character, 

 with its strict integrity and the engaging candour with which he always 

 admitted and even emphasised such difficulties as he had not been able to 

 surmount.' 



We have also to record the regretted death of Dr. Charles Chree on 

 August 12 at the age of sixty-eight. Dr. Chree was superintendent of the 

 Kew Observatory from 1893 to 1925. He was a leading authority upon 

 terrestrial magnetism, atmospheric electricity and related subjects. 



The subject that I have chosen for this address is the Volta eficct. 

 Volta's discovery was made towards the end of the eighteenth century 

 (1792). One form of experiment is as follows : — 



A zinc rod attached to a copper rod is held in the hand. The copper 

 rod is brought into contact with the lower plate of a condensing electro- 

 scope, the top plate of which is touched by the other hand. If the con- 

 nexions are broken and the top plate is raised the gold leaves diverge with 

 negative electricity. This proves that the copper rod was at a negative 

 potential, since the zinc was held in the hand and at the potential of the 

 earth, that is at zero. If the experiment is repeated, but with the copper 

 rod held in the hand and the zinc rod touched to the lower plate, no 

 charge appears, there being a rise from the copper to the zinc accompanied 



