Brown — 3Iechamcal Stress and Macjnetisation of Iron. 177 



in the middle of the solenoid, the circuit was cc^mpleted through the galvano- 

 meter. A current was then sent round the' solenoid in such a direction as to 

 annul the vertical component of the Earth's magnetic field; and when the 

 lower end of the wire was twisted through an angle of 100° no trace of 

 current was produced through the galvanometer ; but, when the circuit of 

 the solenoid was open, and tiie Eartli's vertical field of 0-45 c. g. s. units 

 allowed to go round the wire, a twist of 100° on tlie end of it gave a deflection 

 of 30 divisions on the galvanometer scale. 



In all the experiments mentioned in the first section of this communica- 

 tion, the results are given for a maximum twist on the wire of 100°, so that 

 wlien the thicker wires were under test tliis angle of twist would be within 

 their elastic limits. As in the previous experiments the cyclic curves were 

 obtained hy twisting the lower free end of each wire under test thi'ough 

 a series of steps of 20° from 0° to + 100'^, then from + 100° to - 100°, 

 and again from - 100° to + 100°, thus completing tlie cycle. All the cycles 

 so obtained were plotted on millimetre paper, where on the axis of abscissae 

 one centimetre represented 20° of twist, and on tlie axis of ordinates one 

 centimetre represented 10 divisions on the galvanometer scale ; the total 

 areas of all the curves in sq. cms. were then measured. In order to 

 find the effect of varying the longitudinal stress per unit area on the 

 wire when it was placed in a magnetic field gfeater than tliat of the 

 Earth's vertical force of 0'45 c.g.s. units, a preliminary experiment was 

 made with a No. 16 iron wire ; and it was found that tlie maximum 

 transitory current and the maximum area of the cyclic curve were obtained 

 when the wire was in a magnetic field of between 2 and 3 c. g. s. units : 

 a field of 2'5 units was therefore used. 



A No. 16 iron wire of cross-sectioual area, 20'6 x 10'^ sq. cms., was 

 carefully annealed and suspended in the middle of the solenoid in which 

 was a magnetic field of 2*5 units ; a load of 0'3 x 10'' grammes per sq. cm. 

 was put on the end of the wire, and a complete cycle taken with a maximum 

 twist of 100°. The load was then increased, and a cycle again taken, and so 

 on for six different values of the load up to 2 x lO'* grammes per sq. cm. The 

 cyclic curves were all drawn to the same scale, and the areas in sq. cms. found; 

 the largest transitory current for 100° twist was also observed. The results 

 are here given in Table I. 



[Table I. 

 2g2 



