BiiuWN — Mechanical Stress and Magnetisation of Iron. 187 



With a constant longitudinal stress on each wire of 10^ grammes per 

 sq. cm., the effects of varying the cross-sectional area of the wire, the longi- 

 tudinal magnetisation, and tlie circular magnetisation were now tested ; and 

 to do this four diflferent sizes of wires were employed, viz., Nos. 18, 16, 14, 

 and 12 standard gauge. These wires were properly annealed, and succes- 

 sively tested wlien placed in magnetic fields varying from 0"45 to 14 c.g.s. 

 units, the maximum current density through each wire being at the rate of 

 100 amperes per sq. cm. Each of tlie wires was tested in nine different 

 longitudinal magnetic fields and the total area of the cyclic curve for each 

 set of observations measured : the results obtained are given in Table V., 

 and shown in curves in fig. 6. 



'1'able V. 



The top curve is that obtained with the No. 12 wire of cross-sectional 

 area, 54-7 x 10"' sq. ems., and the lowest curve with the No. 18 wire of 

 cross- sectional area •< 11-7 10"' sq. cms. These curves in fig. 6 are of much 

 the same shape and form as those in fig. 2, which were obtained by twisting 

 the wire in a magnetic field, and measuring tlie transitory current produced. 



From the curves in fig. 6 we see that the circular magnetism rises to a 

 maximum in a weak magnetic field, and then diminishes as the field 

 is increased. It will also be seen that the maximum circular magnetisa- 

 tion in each of the several wires is reached for different values of the 

 longitudinal magnetic field. 



If we pick off from these curves the value of the longitudinal magnetic 

 field corresponding to the highest point on each curve, and plot them as 



