THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. IX. — The Interferometry of Air Carrying Electrical 

 Current / by Carl Barus. 



1. Introduction. — The following experiments, though lead- 

 ing (as was to be anticipated) to negative results, are never- 

 theless sufficiently interesting in their details to deserve to be 

 reported. The object in view was a direct test as to whether 

 a rarefied column of air, through which a current of electricity 

 is flowing, shows any perceptible change of its index of 

 refraction. Such an effect might result from the occurrence 

 of ionization, or from rise of temperature, or finally, in the 

 extreme case, from a possible influence of rapidly moving cor- 

 puscles on the velocity of light traveling in the same direction 

 as the corpuscles. The means of exhaustion at present at my 

 disposal were not sufficient to carry the vacuum much below one 

 millimeter of mercury ; neither were exceptionally large poten- 

 tial differences employed, so that the experiment has not been 

 pushed to a limit at which it might possibly show results. I 

 shall hope to return to the work at some other time. The 

 present paper attempts therefore to do no more than to describe 

 the adaptability of the displacement interferometer for the 

 present purposes. 



2. First Experiments. Apparatus. — At the outset the inter- 

 ferometer was used without other modification than a marked 

 elongation of the arms G M. and GJV, where G is the grating, 

 M (the micrometer) and N the opaque mirrors. The compo- 

 nent beams of light m and n were now one meter in length each, 

 so that a glass tube t, hermetically sealed at its ends with plate 

 glass windows, provided with a tubulure for exhaustion at E 

 (the tube being nearly one meter in length), could be inserted in 

 either beam. The arc light or sunlight euters at the slit S of 

 the collimator C. The direct beam for adjusting the interfer- 



Am. Joue. Sci.— Fourth Series, Vol. XXXIV, No. 200.— August, 1912. 



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