Page:Proceedings of the Royal Society of London Vol 69.djvu/462

442 With alternating magnetic force the phase-displacement is more severe than in the corresponding condition with a rotating magnetic field. At frequency 600 with alternating magnetic force the maximum phase-displacement with a cylinder - 1 cm. diameter is of the order 160. With rotating magnetic force of frequency 1434, for a cylinder of diameter and length each O'l cm., the maximum phase-displacement is 122 and at 717 periods per second it is only 104.

Table II shows that maximum phase-displacement occurs at lower surface induction density the lower the frequency. This is also the case in Table III with alternating magnetic force.

It is unnecessary to publish all the curves of electromotive force obtained. Figs. 8, 9, and 10 refer to a periodic time of 45 seconds, and the corresponding values of induction density can be seen in Table II. Fig. 8 gives the initial stages with small external magnetic force in which the phase-displacements are relatively small, and the No. 1 and 2 electromotive forces are important. In fig. 9, the electro- motive forces of Nos. 1 and 2 coils are relatively smaller, and the phase-displacements are large. In fig. 10, with high external magneti- sing force, the electromotive forces are in phase and of equal relative importance.

It is interesting to note the relation between the average permeability of the iron cylinder, and the rate at which the magnetism is propagated to the centre. When the permeability is small the effects penetrate rapidly. With large permeability in the iron the centre is reached with greater difficulty, hence the diminished value of the induction density at the centre and the increased phase-displacement. With the high limits of induction density the average permeability is small and the effects can penetrate more rapidly.

With intermediate magnetic force at 22'5 seconds periodic time, the electromotive force of No. 1 coil is diametrically opposed to that of No. 3 coil. That is to say, the intensity of induction at the centre of the cylinder is in the same direction as near the surface but of reversed sign.

IV. Longitudinal Variation of Intensity of Magnetic Induction. The preceding remarks refer to the variation of intensity of induction over a section of the cylinder at its centre 2 inches (5 '08 cms.) thick. We have seen how the magnetism is propagated over this section radially towards the centre. It is necessary to deal with the variation of induction density as one proceeds from either end of the cylinder to its centre.

The electromotive forces of each of the coils I, II, III (fig. 1) were observed simultaneously as in the case of coils 1, 2, 3, and by integration the maximum average intensity of magnetic induction over each area has been found. An examination of these curves shows that the effects of induced currents penetrate from each end of the cylinder