120 



Scientific Proceedings, Royal Dublin Societif, 



The lower curve in fig. 9 is that due to a field of 0*45 units, and the 

 upper curve that due to 7 units. From this we see that in botli cases 



o 20 4-0 eo 



Cross-sectional Area of Wire in sq. ems. x 10'. 

 Fig. 9. 



the maximum twist is obtained in these magnetic fields with a wire of cross- 

 sectional area 20-6 x 10"^ sq. cms., and that when the cross-section of the 

 wire is increased about b\ times, i.e., from No. 22 to 16 standard gauge, the 

 twist is increased about 9 times in a field of 0'45 c. g. s. units, and about If 

 times in a field of 7 e. g. s. units. Also for a further Increase in the cross- 

 sectional area of the wire of 2-65 times the twist is decreased about 7 per cent, 

 in the lower field, and about 2-4 per cent, in a field of 7 e. g. s. units. 



Tlie decrease in the maximum twist with the two thicker wires is some- 

 what difficult to account for' ; a similar phenomenon occurs when the cross- 

 sectional area of the wire and the current through it are kept constant, and 

 the longitudinal magnetic field round the wire varied. In the latter case 

 the twist increases up to a certain value of the longitudinal magnetism, and 



' Since the above was written the author has observed that the magnitude of the longitudinal 

 magnetic field required to give maximum twist is diflferent for different-sized wires, i.e. about 

 2-3 c. g. s. units for a No. 12 wire, and 2-8 units for a No. H wire, when the load on each is 

 10* grammes per sq. cm. 



