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L. On the Electrostatic Effect of a Changing Magnetic Field. 

 By J. M. Kuehne*. 



IN the development of Maxwell's Electromagnetic Theory 

 there is implied, i£ not directly expressed, the existence 

 of four distinct mutual effects between electrostatic and 

 magnetic phenomena, some of which have to this day eluded 

 experimental verification. While it may not be said that 

 the value of the theory depends to any very serious extent 

 upon the demonstrability of these assumed effects, yet a great 

 theoretical interest attaches to the question whether the 

 foundations of this theory, which has shown such pheno- 

 menal f ruitfulness in directing research, can be experimentally 

 established. 



The four assumed effects are : — 



1. A moving electric charge produces a magnetic field. 



2. A moving magnetic pole produces an electric field. 



3. A changing electric field produces a magnetic field. 



4. A changing magnetic field produces an electric field. 

 In all four cases the difficulty in the way of experimental 



verification lies in the extreme smallness of the force which 

 is finally to be observed and measured, together with the 

 comparatively very large disturbing forces which must come 

 into play when electric charges and magnetic fields ot\ suf- 

 ficient magnitude are used to make the effect sbufm? for at 

 all observable. This is shown at once by 'the fact that in the 

 denominator of the formula in each case there enters 'he 

 well-known v of Maxwell. 



Of these four effects the first was demonstrated by Rowland 

 in 1876, the third by Eichenwald in 19G3. The ncnver.se 

 effects (2) and (4) have, so far, resisted ail attempts -at ex- 

 perimental proof. The electrostatic effect of a changing 

 magnetic field may be considered as derivable from Row- 

 land's experiment, if the assumption is made that motion of 

 the electrostatic lines of force relative to the aether is not an 

 essential condition of a mutual magnetic and electrostatic 

 effect; but the justification of such an assumption is by no 

 means clear. It was the hope of giving a direct and un- 

 equivocal proof of the existence or non-existence of the last- 

 named effect that prompted the present research. 



Several previous attempts at the solution of this problem 

 have been published, the results, however, being either non- 

 committal or apparently directly contradictory. The first of 

 these attempts was made by Sir Oliver Lodge t in 1889. 



* Communicated by the Author, 

 f Phil. Mag-. [5] xxvii. p. 469. 



