454 On an Effect of Light upon Magnetism. [Mar. 21, 



netic influences. Nevertheless, as is well known, it possesses certain 

 properties which distinguish it from a piece of really virgin iron. In 

 the first place, the magnetisation induced by a force acting in such 

 a direction as to make the marked end a north pole, is greater 

 than that caused by an equal force in an opposite direction. Again, 

 if such a bar be held horizontally east and west (to avoid terrestrial 

 influences), and tapped with a mallet, the marked end at once 

 becomes a north pole. A similar effect follows if the rod be warmed 

 in the flame of a spirit-lamp. Lastly, if it be placed inside a coil and 

 subjected to the action of a series of rather feeble magnetic forces, of 

 equal strength but alternating in direction, the marked end will 

 generally become a north pole, even though the last of the alternate 

 forces may have tended to induce the opposite polarity. 



A rod treated as above described appea.rs to be remarkably sensi- 

 tive to the action of light. When such a rod is placed behind the 

 magnetometer, and illuminated by an oxyhydrogen lamp about 

 70 cm. distant, there occurs an immediate deflection of from 10 to 200 

 scale divisions,* the magnitude of the effect varying in different 

 specimens of iron. As the action of the light is continued, the 

 deflection slowly increases. When the light is shut off, the magneto- 

 meter instantly goes back over a range equal to that of the first 

 sadden deflection, then continues to move slowly in the backward 

 direction towards zero. 



The first quick movement I believe may be due to the direct action of 

 radiation, and the subsequent slow movement to the gradually rising 

 temperature of the bar. With a thick rod (1 cm. in diameter) the 

 slow movement is barely perceptible, extending over only one or two 

 scale divisions in the course of a minute, the spot of light becoming 

 almost stationary after the first sudden jump. With a thin rod the 

 sudden effect is generally smaller, . while the slow after-effect is 

 greater and may continue until the spot of light passes off the scale. 



As a general rule the magnetic effect is such as to render the 

 marked end of the rod a north pole : occasionally, however, it becomes 

 a south pole, but in such cases I have always found that the polarity 

 is comparatively feeble. It may even happenf that the marked end 

 becomes north when certain portions of the rod are illuminated, and 

 south when the light acts upon other portions. This is probably due 

 to irregular annealing and a consequent local reversal of the direction 

 of maximum susceptibility : it indicates that the light effect is local, 

 and is confined to the illuminated surface. In one remarkable speci- 

 men, which happens not to have been annealed at all, the sudden 

 effect and the slow effect are in opposite directions. When the light 



* The magnetometer mirror was 1 metre distant from the scale and each division 

 = 0'64 mm. (^ inch), 

 f This has been observed in two specimens. 



