APR j.] CHARGE IN RAPID MOTION 221 



to the full extent, and collapsing when they touch the 

 boundary, for as long as the radium lasts, which must 

 be many centuries. 



Strutt's apparatus was shown in figure 22, page 177. 



For a good and clear elementary account of the pheno- 

 mena exhibited by radium, especially of all those most 

 important facts which have reference to things other than 

 electrons, such as alpha-rays, the emanations, and the 

 transmutations of matter, the book on The Becquerel Rays 

 and the Properties of Radium, by the Hon. R. J. Strutt, 

 may satisfactorily be referred to, as an introduction to 

 Prof. Rutherford's treatise. 



APPENDIX K. 



Note on the Behaviour of a Charge Moving Nearly at the 

 Speed of Light. 



According to investigations by Larmor in the Phil. 

 Trans., 1897, pp. 228-9, and also according to the investi- 

 gations of Mr. Searle (Phil. Mag., Oct. 1897) a charge 

 does not re-distribute itself on a moving body when its 

 speed becomes great, but the lines of force bend or 

 are deflected towards the equator, without remaining 

 normal to the surface whence they start. Any un- 

 certainty on this head seems to have been due to a 

 natural confusion between the electric force acting on 

 the convected charge and the etherial force which would 

 act on charges at rest, or would cause corresponding ' dis- 

 placement' in force ether; the former must be normal to 

 a perfect conductor, but the latter need not : an assertion 

 in which we may trace some analogy to the fact that in 

 a moving medium rays of light are not perpendicular to 

 their wave fronts. 



