as great as in fig. 6. Apart from phase-displacements we see that the interior portions of the cylinder would be quite useless with regard to induced magnetism, unless very large or very small external magnetising forces were employed.
VII. Experiments were made upon a steel wire O01 inch (0'00254 cms.) diameter, the object being to discover if a true time lag in magnetism exists.[1] Frequencies of 5, 72, and 125 periods per second were tried, and the results obtained were compared with the curve of magnetic hysteresis obtained by the Ballistic-galvanometer method. The curves so obtained would indicate a somewhat higher dissipation of energy with the high frequencies. Now the effects shown in fig. 3 would also be observed in a wire O01 inch diameter if the frequency were 600. The inference is that at 125 periods per second there would still be disturbances due to induced currents. The author inclines to the opinion that the effects associated with time lag in magnetism may be due to induced currents in the wire.
VIII. Lord Kelvin has computed that the earth's magnetism is travelling in the direction of the sun round the earth with a periodic time relatively to the earth of 960 years. The magnetic declination at London had an amplitude of 24 34' 30" W. in 1820. Diurnal variation is observed to the extent of some minutes of arc. The reaction of the induced currents in the cylinder produces a displace- ment of polarity. In a cylinder similar in all respects to the one experimented upon, but having a diameter equal to that of the earth, a periodic time of 960 years would produce similar magnetic and electric events as would be observed in the 10-inch cylinder if the latter could be rotated with a periodic time 12 x 10~ 6 second. This is nearly 2,000,000 times as fast as the fastest speed in these experiments. Diurnal changes in spite of small magnetic force would produce effects confined to the surface of such a cylinder, and would not appreciably disturb an effect due to a periodic time of 960 years. Table II shows that when the intensity of magnetic induction at the surface of the 10-inch cylinder has the value 169, phase differences of the order 44" (360 == 1 period) are experienced with a periodic time of 45 seconds. The magnetic force H per centimetre linear in C.G.S. units would be about 0'5 in this iron, giving an average permeability of about 280. This is a force of the same order as the earth's total intensity.
IX. If we were to work upon smaller and smaller masses of iron until we reached the limit at which all the properties of the original mass were still preserved, we should require enormous speeds of rotation to produce the disturbances examined in this paper. Suppose the diameter of such a mass were of the order O g OOO,0001 cm., then to produce similar effects to those which we have observed at 90 seconds
- ↑ See Hopkinson, Wilson, and Lydall, ' Roy. Soc. Proc.,' TO!. 53, p 352.
