Hart — Action of Light on Magnetism. 67 



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 resist- 

 ance 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 demagnetizing 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 

 demagnetized. The strength of the demagnetizing force 

 required varies according to circumstances: it is generally 

 about one thirtieth or one twenty-fifth of the original mag- 

 netizing force. 



A piece of iron thus treated possesses certain remarkable 

 properties. The magnetization 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. If such a bar be held horizontally east and west 

 and tapped with a mallet, the marked end at once becomes a 

 north pole. Application of heat or instantaneous application 

 of flame causes a similar effect. If subjected to the action of 

 a series of equal, alternating feeble magnetic forces, the 

 marked end generally becomes a north pole, even if the last 

 of the alternating forces may have tended to induce the oppo- 

 site polarity. These were well-known effects, but in addition, 

 Bidwell found that such a rod was remarkably sensitive to the 

 action of light. When placed behind the magnetometer and 

 illuminated by an oxy-hydrogen lamp about 70 cm distant, 

 there occurred an immediate deflection of from 10 to 200 scale 

 divisions, the magnitude of the effect varying in different 

 specimens of iron. The magnetometer mirror was one meter 

 distant from the scale and each division=0*64 mm . In these 

 experiments, as the action of the light was continued, the 

 deflection slowly increased. When the light was shut off, the 

 magnetometer instantly, went back over a range equal to that 

 of the first sudden deflection, then continued to move slowly 

 in the backward direction toward zero. With a thick rod the 

 slow movement was barely perceptible. He believed the first 

 quick movement to be due to the direct action of radiation, 

 and the subsequent slow movement to the gradually rising 

 temperature of the bar. In several cases, the magnetism was 

 of the opposite kind, and sometimes became north when cer- 

 tain portions of the rod were illuminated and south when the 

 light acted upon other portions. This was probably due to 

 irregular annealing and a consequent local reversal of the 



