Brown — Mechanical Siress and Magnetisation of Iron. -483 



In plotting the results of these experiments on rigidity and electrical 

 conductivity, an arbitrary scale of hardness has been taken ou the axis of 

 abscissae, that is, the load in grammes per sq. cm. which was on the end of the 

 wire when it was heated has been taken as a measure of the liardness. 



Against this, on tlie axis of ordinates tlie simple rigidity has been plotted 

 in the one case, and in the other case the percentage electrical conductivity 

 compared with pure copper. 



The conductivity curve here given is similar to those obtained by 

 Strouhal and Barus' in their work on the specific resistance and temper of 

 steel wire. 



In order to observe the amount of twist on the end of the wire when it 

 is subjected to different longitudinal loads, and when placed in different 

 longitudinal magnetic fields — tlie maximum current sent through the wire in 

 all cases was at the rate of 100 amperes per sq. cm. — the same long 

 solenoid with accessories was used as was employed in previous work.^ The 

 wire under test was fixed so as to hang vertically, and as near as possible in 

 the middle of the solenoid, whose total length was 237 cm. ; the distance 

 from the mirror on the vibrator at the lower end of the wire to the 

 millimetre scale was 116'5 cm., and for every millimetre of deflection on 

 the scale, the end of the wire was twisted through an angle of 88 seconds. 



With a certain load on the end of the wire and a given longitudinal 

 magnetic field round it, an electric current was sent through tlie wire, and 

 its value slowly increased up to the maximum of 100 amps, per sq. cm. — so 

 as to give a constant circular magnetism — and the steady deflection read off 

 the millimetre scale ; the current was then gradually diminished to zero, 

 reversed, and again slowly increased to the maximum, and the scale-reading 

 again observed on the other side of the zero ; the mean of these two readings 

 was then taken as the true value of the twist for that magnetic field. 



The longitudinal load on the wire being kept the same, a similar process 

 was gone through for twelve different magnetic fields up to a maximum of 

 14 c.g.s. units. 



The longitudinal load on the wire was now increased, and a series of 

 observations were made, similar to what was done for the first load, and so on for 

 five different values of the load. Before doing these, however, the wire, which 

 had been strongly magnetised during the previous experiments, had to be 

 carefully de-magnetised each time ; this was a somewhat troublesome 

 operation, but was overcome by means of a reversing key in the circuit of 

 the solenoid ; by continually reversing the diminishing current through 

 the solenoid, and decreasing the load on the end of the wire and the 



1 Wied. Ann., 1883, vol. xx., p. 621. 



2 Scient. Pioc. Roy. Dub. Soc, 1909, toI. xii., pp. 115 and 183. 



