870 



II. XAGAOKA. 



nickel is determined by the direction of the spiral of compression. 

 Sir William Thomson described these phenomena in the following 

 language.* 



"To avoid circumlocutions suppose the iron or nickel wire to be 

 vertical, and the magnetizing current to be in the opposite direction to 

 that of the motions of the hands of a watch held with its face up. The 

 undisturbed magnetization is downwards. Now suppose a right- 

 handed twist to be given to the wire. Its elongational spiral is right- 

 handed, and its contractional spiral is left-handed, [f the substance 

 is iron, the lines of magnetization become left-handed spirals ; if 

 nickel, right-handed. Now a downward current, in the downwardly 

 magnetized wire, would, by the superposition of circular magnetiza- 

 tion in the direction opposite to that of the hands of a watch, cause 

 the lines of magnetization to become left-handed spirals. Hence the 

 sudden right-handed twist induces in iron a current upwards, in 

 nickel a current downwards. Thus we have the following simple 

 spécification for the directions of the induced longitudinal currents in 

 the two substances, without reference to " up" and "down." 



" From any point. P, on the surface of the wire draw samewards 

 parallels to the current in the nearest part of the magnetizing solenoid, 

 and to the direction of the induced longitudinal current. Draw a 

 helix through P making an acute angle with each of these lines. 

 This helix is of same name as the elongational helix fr,v iron, and 

 as the contractional helix for nickel." 



Another set of experiments was tried by twisting the nickel wire 

 in a constant magnetizing" field, different amounts of twist being taken 

 in succession. The results are shown graphically in Fig. X. 

 For the wire of 0.5 mm. radius: — 

 Curve I. for £= 5.0 



* See note to uiy former paper, Phil. Map., Jan. 1890. 



