66 Hart — Action of Light on Magnetism. 



Art. XI. — The Action of Light on Magnetism : by Joseph 



Hall Hart. 



The electro-magnetic theory of light assumes the existence 

 of electro-magnetic waves of the required frequency in the 

 ether and of their identity with ordinary light-waves. In con- 

 sequence of the alternate electric displacements there should 

 be an alternating magnetic field and we might expect the 

 demagnetization of iron by light as a result, if the intensity 

 of the light were gradually diminished. We have, however, 

 little trustworthy evidence of the action of light on magnetism. 

 The contradictory results obtained and the smallness of the 

 effect have prevented the attainment of experimental results 

 sufficient for definite conclusions on the subject. In particular, 

 the action of light on the magnetism of iron bars presents a 

 field in which, if ordinary care is taken, positive results should 

 be obtained. 



Several experimenters in the early part of the present cen- 

 tury tried to magnetize iron and steel by the action of light. 

 Morichini,* Christief and Mrs. Somerville^: obtained positive 

 results from a series of experiments, while P. Riess and L. 

 Moser,§ and John W. Draperf obtained negative results from 

 precisely similar experiments, and the thing has been generally 

 regarded as impossible. Under ordinary circumstances there 

 can be little doubt that this is the case, but, according to Bid- 

 well,^ in a paper before the Physical Society in 1889, if a cer- 

 tain initial condition is fulfilled, we can find evidence of the 

 action of radiation upon the magnetism of iron, and the ful- 

 fillment or non-fulfillment of this condition explains the 

 diverse results of previous experimenters. 



The condition is that the susceptibility of the bar AB to be 

 operated upon shall be greater (or less) for a magnetic force in 

 the direction AB than for an equal one in the direction BA. 

 Iron bars having this property were obtained by the following 

 method : A piece of soft iron rod, which may conveniently be 

 10 or 12 cm long and from - 5 to l cm in diameter, is raised to a 

 bright yellow heat and slowly cooled. When cold, it is placed 

 inside a solenoid, through which is passed a battery current of 

 sufficient strength to produce a field of about 350 or 400 

 C. GL S. units. The iron when removed from the coil is found 

 to be permanently magnetized, and its north pole is marked 



*See Christie. f Phil. Trans., 1826, p. 219. 



% Phil. Trans., 1826, p. 132. 



§ Annales de Chernie, xlii, 304. 



|| Journ. of Franklin Institute, Eeb. 1835. 



«|J Proc. Phys. Soc, London, April, 1889, p. 455. 



