492 



Scientific Proceedings^ Royal Dublin Society. 



referred to, the results are given of a test on au iron wire when the current 

 through the wire was kept constant, and the longitudinal magnetic field round 

 it varied, and it was found that the maximum twist occurred in a magnetic 

 field of about 20 units. The wire used by them was 21 cm. long and 

 0'98 mm. in diameter ; the current sent through it was 6 amperes, or at the 

 rate of nearly 800 amperes per sq. cm. ; that is, about eight times the current 

 density used in most of the experiments described in this paper, and the 

 maximum twist found by Nagaoka and Honda occurred in a magnetic field 

 about eight times that found by the present writer witli a wire about 1'6 

 mm. in diameter. 



In order to test experimentally how the magnitude of the magnetic field — 

 in which the maximum twist occurs — varies when currents of different value 

 are sent through the wire, a No. 17 iron wire was taken (in the physical 

 state in which it came from the manufacturer) and prepared for testing in 

 the usual manner. The wire was placed in the solenoid, with a load on the 

 end equivalent to 10* grammes per sq. cm., and the twist measured when the 

 wire was subjected to twelve different longitudinal magnetic fields. Three 

 sets of observations were taken when the current through the wire was 1'5, 3, 

 and 6 amperes respectively, or when the current densities were 95, 190, and 

 .380 amperes per sq. cm. The results obtained are shown in Table VII. 



Table VII. 



