412 C. Barus — Torsional Magnetostriction in Strong 



and both wires are thus subject to the same torque. The 

 change of rigidity due to the transverse magnetization is 

 observed by the mirror, m, reflecting a beam of sunlight upon 

 a wall 5 meters distant. Hence l cm of deflection 

 corresponds to about -001 radian or -057°. The 

 mirror is provided with vanes, vv, dipping in an 

 annular trough (not shown) to deaden vibration, 

 as well as to carry a current from E to v for the 

 circular fields. The rod, e, is suitably provided 

 with a pin and slot arrangement, so that the 

 wires may be kept under the definite tension of 

 the weight, W. 



The core of the electromagnet, A, was of 

 thick gas pipe about 37 cm long, 5"5 cm in diameter 

 and - 3 cm thick, the longitudinal (vertical) air gap 

 being about 'S cm broad. It was wound from end 

 to end with about 120 turns of wire. A fine 

 vertical glass tube (not shown) about m l cm in 

 bore was symmetrically secured within the gap 

 running from end to end. Through this tube 

 the wire, ah, was threaded to prevent adhesion 

 to the sides of the jaws of the electromagnet. 

 This occurrence in fact constitutes the chief 

 difficulty in the experiments. Deflection of the 

 wire toward either jaw is liable to be accom- 

 panied with spurious rotation. On the other hand too fine a 

 tube interferes with the motion fractionally. In later experi- 

 ments I discarded the glass tube in favor of a more carefully 

 centered free wire. 



During the tests with circular magnetization the lower wire 

 canwing the current was submerged in a wide glass tube of 

 flowing water. 



The field H, within the air gap of the helix, measured bal- 

 listically with the aid of an earth inductor, had approximately 

 the following c. g. s. values for different currents, C, in amperes. 

 An attempt to secure sharp absolute values was needless. 



C= 1 2 3 4 5 6 1 8 9 10 11 12 13 



5=1200 2700 4300 5700 7150 8500 10000 11100 12000 12650 13200 13700 13950 



Between 2 and 7 amperes in the helix the variation of .ZTwith 

 C is very nearly linear. Below this it is slightly accelerated, 

 above rapidly retarded. Since all fields were made from zero, 

 the effect of hysteresis could be disregarded as was specially 

 tested. The field here referred to is that within the air gap 

 and found by differential experiments. If B 1 be the induc- 

 tion within the gap and JB 2 the total induction, including the 



* Barus: this Jour., (3), xxxiv, p. 176, 1887. 



