490 



Dr. H. A. Wilson. On the Electric Effect of [May 18, 



that in both sets of tertiaries (right and left) there is a tendency to 

 curve downward — and this is well known to occur in normal secondary 

 roots exposed to light, and is another instance of the assumption by the 

 tertiaries of characters hitherto associated only with secondary roots. 



The facts above given prove that when the primary root is removed 

 and a secondary root assumes its place, the tertiary roots take on the 

 character of normal secondaries. It may be believed, therefore, that 

 the existence of statoliths in normal tertiary roots is a provision 

 enabling them to assume diageotropic growth in case of injury to the 

 primary root. This, though appearing a bold conclusion, does not 

 involve an adaptive action different in principle from the well-known 

 assumption by secondary roots of the characters of the primary root. 



We desire to express our thanks to Mr. T. Elborn, Assistant in the 

 Cambridge Botanical Laboratory, for the help he has given us during 

 the course of our research. 



" On the Electric Effect of "Rotating a Dielectric in a Magnetic 

 Field." By Hakold A. Wilson, M.A., D.Sc, Fellow of 

 Trinity College, Cambridge. Communicated by Professor 

 J. J. Thomson, F.E.S. Eeceived May 18, — Eead June 2, 

 1904. 



(Abstract.) 



It was shown by Faraday in 1831 that an electromotive force is 

 induced in a conductor when it moves in a magnetic field so as to cut 

 the lines of force. The object of the experiments described in this 

 paper was to see if a similar electromotive force is induced in a 

 dielectric when it moves in a magnetic field. 



According to Maxwell's electromagnetic theory, as developed by 

 H. A. Lorentz and Larmor, such an electromotive force should be 

 induced in a dielectric, and should be equal to that in a conductor 

 multiplied by the factor 1 - K -1 , where K is the specific inductive 

 capacity of the dielectric. 



The method employed was to rotate a hollow cylinder of ebonite in 

 a magnetic field parallel to the axis of the cylinder. The inside and 

 outside surfaces of the cylinder were provided with metal coatings with 

 which electrical contact was made by sliding brushes. The inside 

 coating was connected to earth, and the outside coating to one pair of 

 the quadrants of a sensitive quadrant electrometer, the other pair of 

 quadrants being connected to earth. The magnetic field was then 

 reversed, so reversing the induced electromotive force in the ebonite. 

 The resulting electric displacement was measured by means of the 



