496 Scientific Proceedings, Royal Dublin Society. 



When the wires are of different diameters, the maximum twist occurs in 

 magnetic fields of different strengths ; in the top curve in fig. 6, which gives 

 the results obtained with the thickest wire, the maximum twist takes place 

 in a field of 4 units, and the lowest curve which was obtained with the 

 smallest wire has its maximum twist in a field of about 2-3 units. By- 

 plotting the cross-sectional areas of the wires as abscissae, and the corre- 

 sponding values of the twist, for any field, as ordinates, the points all lie 

 very approximately in a straight line in each case ; and when the cross- 

 sectional area of the wire is increased about nine times, the increase in the 

 twist is larger as the field is increased. Thus in a magnetic field of 3 

 units the twist is doubled, and in a field of 6 units the twist is increased 

 about 21 times, and about 2| times in a field of 14 c.g.s. units. 



In an application of Kirchoff 's theory to the Wiedemann effect, Nagaoka 

 and Honda' have shown theoretically that for a given longitudinal current 

 through the wire, the twist of the free end is inversely proportional to the 

 square of the radius. In order to test this experimentally, three wires were 

 taken of different diameters (viz. Nos. 15, 16, and 17), all in the same 

 physical condition, as delivered by the manufacturer, and tested in twelve 

 different longitudinal magnetic fields. The wires were each 226 em. long, 

 the load on each was 10^ grammes per square cm., and the current through 

 each was 1'5 amperes or of current densities 95, 72"8, and 57'5 amperes per 

 sq. cm. respectively. The tests were made in the usual way, that is, with a 

 given longitudinal magnetic field round the wire : the current through it was 

 raised to 1*5 amperes, and the twist or deflection on the scale read off; the 

 •cm-rent was then diminished to zero, reversed, again raised to 1*5 amperes, 

 and the twist again observed ; the mean of the two readings was then taken 

 as the true twist for that magnetic field. The same process was gone 

 through for fields up to 14 c.g.s. units. 



i Phil. Mag., 1902, 6th Ser., vol. iv., p. 6S. 



