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Scientific Proceedings, Royal Dublin Society. 



value of the spiral field 8' was chosen, and the longitudinal applied field 

 H' and the applied circular field F were set so as to give this value of 

 S' for a given value of the pitch-angle a where tan a = H'jF. The notations 

 8' and R' are used here to denote the applied magnetic fields, postponing the 

 question of their relation to the actual fields 8 and IT until we consider the 

 demagnetizing effects of the ends in the later sections of the paper. The 

 process of anhysterization was carried out and the deflection read. This 

 method should produce the same intensity of magnetization along the 

 resultant spiral field while a alone varies, thus complying with the mathe- 

 matical conditions provided the tube is magnetically isotropic. The observa- 

 tions were taken according to the scheme given in Table I, which also gives 

 the observations for 8' = 14. It is easily seen that the deflections vary as 

 sin2a. 



Table I. 



Twist of a steel tube in a resultant spiral magnetic field S' = 14 for 

 varying pitch-angle a. 



Applied longitudinal field = S' sin a. 



Applied circular field = 8' cos a. 



Current in amperes in central wire = 3S' cos a. 



The process of anhysterization causes a slight disturbance of the zero of 

 the mirror, owing to the to-and-fro vibration which accompanies the appli- 

 cation of the alternating magnetic field, so that a reading of the zero had to 

 be taken after each observation. This, of course, requires the complete 

 demagnetization of the tube by the method already described. The shift of 

 the zero due to this cause was very slight, usually of the order 0T to 0"2 mm. 

 A very striking feature of the twist is its absolutely dead-beat quality, the 

 diffraction band moving immediately to its new position on the application of 

 the magnetic fields. The results for a series of observations for spiral fields 

 S' = 14, 20, 30, 40, and 60 c.g.s. units are shown in fig. 2. A complete set 



