Brown — Mechanical Stress and Magnetisation of Iron. 115 



current was reversed and the same steps taken to 206 amperes and down to 

 zero, when it was again reversed, and hj the steps increased up to two 

 amperes again, thus finishing the complete cycle. The twist on the wire 

 lags on the current going through it, and when the currents in amperes 

 through the wire are plotted against the twist or deflections on the scale a 

 very symmetrical cyclic curve is obtained. 



The maximum current of 2'06 amperes used through this No. 16 wire 

 was at a current density of 100 amperes per sq. cm., and was not large 

 enough to appreciably change the physical condition of the wire. A cycle 

 was taken when the wire was suspended in the vertical component of the 

 Earth's magnetic field 0'45 c.g.s. unit, and with a load on the wire of 

 10^ grammes per sq. cm. The observations were then plotted on millimetre 

 paper, where 2 cms. represented one ampere on the axis of abscissae, and 

 1 cm. on the axis of ordinates represented ten divisions of twist on the scale, 

 the distance from the scale to the mirror being 114 cms. and 700 divisions 

 on the scale being 44 cms. The area of the complete cyclic curve so obtained 

 was found to be 23"5 sq. cms, and the maximum twist for 2'06 amperes was 

 36'8 scale divisions. When the load on the wire was doubled and another 

 cycle taken in the same field, and with the same maximum current, the 

 area of the cyclic curve was increased by 3'5 sq. cms., and the maximum 

 twist increased by about 5 scale-divisions. (H. Gerdien' has shown that the 

 cyclic variation in the torsion of an iron wire produces a cyclic variation in 

 the_longitudinal magnetic moment.) 



In order to test the effect of a stronger magnetic field, a solenoid was 

 now put round the wire when still in a vertical position. This solenoid 

 was 236 cms. long, and 2 cms. internal diameter, and consisted of 7707 turns 

 in 4 layers of number 20 d.c.c. copper wire wound uniformly over the whole 

 length, the resistance of the coil being 17 ohms at 12°C, and the magnetic 

 field inside the solenoid being about 41 c.g.s. units per ampere. 



When the solenoid was in position, the wire projected from the ends by 

 about 2 cms. at the top, and 8 cms. at the bottom. This method of getting 

 a magnetic field does not allow the wire — as here arranged — to be in a 

 uniform magnetic field throughout its entire length, so that results got by its 

 means are not quite comparable with those obtained when the wire is in a 

 uniform magnetic field such as the vertical component of the Earth's magnetic 

 force. The solenoid was, however, tested experimentally by means of an 

 exploring coil, and the internal magnetic field was found to be perfectly 

 uniform along the whole length to within 5 cms. of each end, so that in the 



I Ann. d. Physit, 1904, pp. 51-86, 



