144 DISPLACEMENT INTERFEROMETRY APPLIED TO 



slow vibration of the needle. It follows, therefore, that in spite of the large 

 masses used, the radiant forces are by no means negligible, even in the pres- 

 ent cumbersome experiment.* 



Finally, it is interesting to note, since the torque is T = 0.265, the angle 

 6 = 0.00175, and the daily detorsion 5 = 0.8 cm., that energy is being dissipated 

 at the rate of 70/2, or 0.00014 erg per day, by this slight torsional strain alone. 

 Much more is doubtless released by the decay of the intense traction (wire- 

 drawn) which the wire carries. If the metal were not exceedingly opaque it 

 would probably be phosphorescent. 



II. THE TORSIONAL MAGNETIC ENERGY ABSORPTION OF AN IRON 



CONDUCTOR. 



110. Apparatus. The relations of torsion and magnetization have been 

 studied by Wiedemann, Auerbach, and many others since, chiefly in longitudi- 

 nal fields. The torsion effect produced by a circular field is very small and 

 difficult to ascertain ; therefore, I thought it of interest to make some measure- 

 ments of this kind, using the displacement interferometer and achromatic 

 fringes. The results were very definite and would easily have admitted of 

 precision. The apparatus is shown in figure 181, where i-m, H TJTI 

 AB is a thin low-carbon steel tube, effectively 55 cm. long, j 

 having an average diameter of 0.875 cm - and walls 0.076 

 cm. thick. 



The tube is firmly clutched below by a clamp, but free 



181 



above. It carries the mirror mm', which is a strip of thin 



plate glass, silvered, and slightly adjustable about vertical 



and horizontal axes. The ends receive the component 



rays of the interferometer, so that any slight rotation 



of mm' about the vertical axis is at once registered by the displacement of 



fringes. Finally, a strong electric current may be passed through the length 



of the tube, entering at A and leaving by the mercury cups C. The current 



must be reversible at pleasure. 



ill. Observations. The fringes are displaced (i. e., the tube receives mag- 

 netic set) immediately after closing the circuit. Closing it any number of 

 times thereafter is ineffective, to the fraction of a fringe. There is practically 

 no temporary effect. On reversing the current, the fringes are markedly 

 displaced in the opposite direction, again, to hold the new position, however 

 often, the current is made and broken thereafter. The effect of reversal is 

 static and may be repeated indefinitely. 



To obtain a temporary effect, I surrounded AB with a massive iron tube, 

 about 6 inches long and 2 inches in diameter, clamped at the top B, but other- 

 wise free from it. Even now, with currents up to 20 amperes, I observed no 

 temporary effect in excess of the quiver of the fringes. 



*The relatively large temperature coefficient of torsional viscosity is also effective. 

 Similarly, the thermal changes of rigidity reciprocate with the concealed torsional stress. 



