1889.] On an Effect of Light upon Magnetism. 453 



III. " On an Effect of Light upon Magnetism." By SHELFORD 

 BIDWELL, M.A., F.R.S. Received March 11, 1889. 



Several experimenters in the early part of the present century 

 tried to magnetise iron and steel by the action of light,* but I do not 

 know of any recent attempts in this direction, and of late years the 

 thing has been generally regarded as impossible. Under ordinary 

 circumstances there can be little doubt that this is the case, but, if a 

 certain condition is fulfilled, we might, I think, expect to find some 

 evidence of the action of radiation upon the magnetism of iron. 



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. This paper contains 

 a short preliminary account of a series of experiments which have 

 been undertaken with iron bars having this property. Much yet 

 remains to be done, which will require a considerable amount of time, 

 and for which special apparatus must be constructed. In the mean- 

 time, the results already obtained appear to possess sufficient interest 

 to justify their publication. 



The method of preparing the bars is as follows : A piece of soft 

 iron rod, which may conveniently be 10 or 12 cm. long and from 0'5 to 

 1 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.G.S. units. The iron when removed from the coil is found 

 to be permanently magnetised, and its north pole is marked for the 

 sake of distinction with red sealing-wax varnish. The bar is then 

 supported horizontally and in an east and west direction behind a 

 small reflecting magnetometer, and over it is slipped a coil, which is 

 shunted with a rheostat, the resistance of which can be gradually 

 increased from to 26 ohms. The coil can be connected by a key 

 with a single battery cell, which is so arranged as to produce a 

 demagnetising force inside the coil. The resistance of the rheostat 

 is slowly raised, so that more and more current passes through the 

 coil, the battery key being alternately lifted and depressed until the 

 magnetometer indicates that the iron bar as a whole is perfectly 

 demagnetised. The strength of the demagnetising force required 

 varies according to circumstances : it is generally about one-thirtieth 

 or one-twenty-fifth of the original magnetising force. 



After this treatment the iron rod does not differ, so far as ordinary 

 tests will show, from one which has never been submitted to mag- 



* Chrystal, ' Encycl. Britann.,' vol. 15, p. 274, mentions the following 

 Morichini, Mrs. Somerville, Christie, Riess, and Moser. 



