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XLIY. A Fundamental Experiment in Electricity. By Prof. 

 A. M. Worthington, C.B., F.R.S., Royal Naval Engi- 

 neering College, Devonport *. 



THE object of the experiment which I desire to record 

 was to ascertain whether any difference conld be 

 detected between space at a high electrical potential and 

 space at a low potential, quite irrespective of the existence 

 of any electric field in the space in question. 



Any easily noticeable difference was not to be expected. 

 Such would probably have been long since discovered; for, 

 if it exists, it can only correspond to some small residue of 

 energy not accounted for in the well-established relations 

 of the electric field. 



To most people the question may seem to have been settled 

 once for all by the experiments recorded by Faraday in his 

 ' Experimental Researches/ vol. i. p. 1174, in which he 

 describes how he constructed a hollow, insulated, conducting 

 cube, inside which he went, "and lived in it ; and using 

 lighted candles, electrometers, and all other tests of electrical 

 states, could not find the least influence upon them, or indi- 

 cation of anything particular given by them, though all the 

 time, the outside of the cube was powerfully charged, and 

 large sparks and brushes were darting off from every part of 

 its outer surface." 



It must be observed, however, that what Faraday was 

 looking for was some sign of what he called an absolute 

 charge — i. <?., as he explains, a charge existing by itself and 

 not connected by lines of induction with an equal charge of 

 opposite sign ; and it will be noticed that the tests he mentions 

 were tests of " electrical states." He does not seem to have 

 employed tests of other physical conditions. 



It therefore seemed to me worth while to examine whether 

 any difference could be detected in the velocities of light 

 passing through two tubes, the interior of one of which was 

 at a high, and of the other at a low potential. 



The arrangement employed was that of a Rayleigh interfero- 

 meter, and is shown in the diagram. 



The light of a Cooper-Hewitt mercury- vapour lamp A 

 passes through a fine vertical slit B, at the focus of a colli- 

 mating lens C (This lens was an achromatic object-glass of 

 126 cm. focal length.) The light then passes, as a beam 

 of parallel rays, to the two fine vertical slits D D, and thence 

 through the two open brass tubes E E, each 5 feet (152*4 cm.) 

 long, and separated by a partition of ebonite about 2 mm. 



* Communicated by the Author. 



