CHANGK OF ELASTIC CONSTANTS OF FERROMAGNETICS. :>? 



the resistance of air, the Foucaiill current etc., tlie jjcriod of 

 oscillation was made very long by using u weight with consider- 

 able moment of inertia. A rectangular brass bar (42.80 x 0.895 x 

 l.'.M() cm."') was horizontally fixed to the lower end of the lower 

 rod, at its middle point. Two e(][ual cylindrical weights (each 

 ()ô7 gr.) made of had could be fixed symmetrically on the bar at 

 any desired distance from the middle. Besides, two ecjual cylindri- 

 cal weights (1()71 and 1(540 gr.) of lead could l)e put on the bar 

 with their axes coincidiuir with that of the rod. Suitable combina- 

 tions of these different weights enabled us to adjust the periods of 

 oscillations for different tensions to convenient values. A liirlit 

 mirror was lixed to the lower rod. The image of a horizontal 

 scale placed in Iront (jf the arrangement was observed with a 

 telescope in the usual manner. 



To start the torsional oscillation of the wire, a lead tube was 

 directed to one end of the horizontal l)ar. The tube going to the 

 observer was held by his hand ; the oscillation was started by 

 l)lowing through the tube, while observing through the telescope. 

 The amplitude of the oscillation could be increased or diminished 

 at will, by blowing with suitable force on the bar in a suitable 

 phase of its oscillation. In this way, the equality of the amplitude 

 could easily be effected within 1 mm. uf the scale. It was usually 

 Ü cm. with a scale distance of 1.40 m., which corresponds to an 

 amplitude of about 1°. 



It was necessaî'y to ])rotect the oscillating system from distur- 

 bance due to air currents. 



Our procedure was as follows : — The wire to be tested was- 

 first demasrnetized bv reversals, the zero of the scale set to the 

 position of ecpiilibriuni ; a iield was applied, and then oscillation 

 started. Care was taken to adjust the amplitude lor dillerent fields 



