on Rods of Iron, Nickel, and other Metals. 385 



ampere was H= - — ^ — — = 97'1 C.G.S. units. The coil 



r 50 x 10 



was fixed vertically on a strong board. Under its lower 



end, at a distance of 4 centim., a brass frame was fixed 



carrying a mirror on an axle, the axis of which coincided with 



that of the coil. The rods to be examined were the same length 



as the coil but elongated at each end with nonmagnetic wire ; 



these were passed through the coil and attached at one end to 



the axle of the mirror, which was used in conjunction with a 



telescope and scale. Any minute torsion of the rod could be 



accurately measured. The scale was placed at a distance of 



226 centim. from the axis of the mirror, the wire was passed 



through collars within the tube ; these kept it in a central 



position and prevented any movement sideways. By means of 



a liquid resistance in the circuit the current was regulated, so 



that the value of H could be easily changed. 



4. The first iron rod which was tested was 50 centim. long 

 and 0*162 centim. in diameter, well annealed by being placed 

 in a tube furnace and raised to a bright red heat and cooled 

 slowly. (All the metals were supplied by Messrs. Johnson 

 and Matthey, and may be considered as pure as can be pro- 

 cured.) The reading of the scale as seen in the telescope 

 was brought to zero, and then the rod was magnetized with a 

 magnetizing force H of 21*5 C.G.S. units. A rotation of 

 0*1 centim. was produced in a direction with clock-hands to a 

 person looking from the fixed end down the rod ; when the 

 current was broken it returned instantly to its initial position. 

 A very delicate thermometer was put from time to time in 

 the usual position of a rod under examination and then the 

 current was put on as for an experiment ; no change of tem- 

 perature could be detected, so that the effect produced was 

 due to the magnetizing force alone. 



5. The rod in (4) was replaced by one which had received 

 a permanent torsional set of seven revolutions. To give this 

 amount of torsion the rod was fixed at one end and then the 

 other was rotated through over eight complete revolutions, it 

 was finally left with a twist of seven revolutions. The twist was 

 with clock-hands to a person looking down the rod from the 

 fixed end as an axis of the couple ; it was then subjected to a 

 force H = 21'5. This instantly produced a further torsion in 

 the same direction as the initial torsion of 5 scale-divisions. 

 The scale (divided into centim. and millim.) was in this and 

 all the following experiments 180 centim. from the mirror. 

 The effect was carefully examined by several observers and 

 always reproduced when required. In subsequent experiments 

 I found that when the rotation was large it could be readily 

 made visible by the addition of a light pointer attached to the 



Phil Mag. S. 5. Vol. 32. No. 197. Oct. 1891. 2 D 



