514 Scientific Prnceedings, Roi/al Duhlin Society. 



each case occurring in the same longitudinal magnetic field of ahout 

 13 c.g.s. units. This also is different from what takes place with an iron 

 wire when it is tested in the same way and under the same conditions, for 

 with iron the longitudinal magnetic field in which the maximum twist occurs 

 is different for different-sized wires.' In the case of nickel wires, as will be 

 seen from Table VI, when the cross-sectional area of the wire is increased 

 about 4'3 times, the increase in the twist is larger as tlie longitudinal 

 magnetic field is increased ; thus in a field of 13 units (the field in which 

 the maximum twist occurs in each ease) the twist is increased 12'6 per cent., 

 in a field of 30 units 16"7 per cent., and 19'3 per cent, in a field of 50 e.g.s. 

 units. 



It has been shown — with iron wires — that the twist was directly pro- 

 portional to the current-density in the wire for any given longitudinal 

 magnetic field round it. Table V above gives the results of tests made with 

 a No. 16 nickel wire, with a longitudinal load on it of 0'5 x 10^ grammes per 

 sq. cm., when different currents were sent through the wire. Experiments 

 were now made with three wires of different diameters, and the same value of 

 eurrent through each — that is, the current densities were 100, 72'4, and 55-2 

 amperes per sq. cm. respectively. The three wires were tested in the state of 

 magnetic softness, represented by Ho on our arbitrary scale, and were Nos. 

 16, 17, and 18, s.w.g. The length of each wire was 226 cms., and 

 the longitudinal load on each 10^ grammes per sq. em. The results obtained 

 are given in Table VII. 



' Scieiif. Pioc. Eoy. Dub. Sop., vol. xii.. p. 49o. 



