422 Scientific Proceedings, Royal Dublin Society. 



The Magnetization of a Steel Tube in a Spiral Magnetic Field. 



Influence of the demagnetizing factor. — Experiments on magnetic phenomena 

 which have to be performed with linear rods are subject to the grave dis- 

 advantage that the magnetization at the ends sets up an opposing field to the 

 applied force, so that neither the resulting field nor the magnetization can be 

 uniform. This effect may have an important influence on the twist in the 

 spiral magnetic field, and may be the cause of the departure from the linear 

 law shown in fig. 2. This can only be demonstrated by showing that the 

 linear law holds strictly if a correction is introduced for the magnetization of 

 the ends, which is usually done by means of the demagnetizing factor N, 

 approximately constant, where IT - NI = H, the average magnetizing force 

 existing in the iron and referring the values of I to H instead of IT . 



Unfortunately there are no reliable data regarding the demagnetization 

 effects in a steel tube, and special methods had to be devised to evaluate this 

 constant from observations on the magnetization. The values of I for different 

 values of a longitudinal field IT and a spiral field »S" are therefore required. 



Magnetometer measurements. — The measurement of the intensity was 

 carried out in the usual way by the one-pole magnetometer method. A com- 

 plexity is introduced by the fact that the spiral field is produced, in part, by 

 the current through the central wire in the steel tube. The magnetic field 

 clue to the current is, of course, parallel to and varies the magnitude but not 

 the direction of the control field at the magnetometer. This, however, merely 

 requires a determination of the value of the control field in each observation 

 by means of an auxiliary coil in series with a calibrated milammeter. The 

 whole series of measurements was affected by small variations in the zero, due 

 to the neighbourhood of magnetic material, which had to be eliminated by the 

 method of taking observations, before a- consistent series could be obtained. 

 The most troublesome effect was in the variation of the vertical component of 

 the earth's field due to extraneous magnetic material consequent on the 

 variety of magnetic fields to which it was subjected. The current in the 

 compensating coil had, therefore, to be varied accordingly. The range of 

 variation was about O05 c.g.s. units. 



The only other point which may be noticed is the determination of the 

 equivalent position of the poles of the steel tube. Ewing states that the 

 position in which the deflection is a maximum may be taken as the position 

 in which the magnetometer and one pole are in the same horizontal plane. 

 This is only approximate. The difference in the present observations amounted 



