RESEARCHES IN MAGNETISM. 
569 
increase in the speed of signalling becomes possible when copper is substituted for 
iron in a line whose electrostatic capacity and resistance are kept unaltered ; and that 
the much more rapid electric impulses which traverse a telephone circuit often become 
muffled beyond recognition when iron takes the place of phosphor-bronze.] 
§ 52. Time Lag in Magnetisation. —A few miscellaneous points will now be noticed 
which presented themselves in the course of these experiments. 
I have already said that the hysteresis which has been described as occurring in the 
relation of magnetisation to magnetising force is of a static character. Some 
evidence was, however, given that in addition to much static hysteresis there is a 
small amount of viscous lagging in the changes of magnetism which follow changes 
of magnetising force. I repeatedly observed that when the magnetising current was 
applied to long wires of soft iron, either gradually or with more or less suddenness, 
there was a distinct creeping up of the magnetometer deflection after the current had 
attained a steady value, as measured by the deflection of the galvanometer through 
which it passed. This action was sometimes so considerable as to oblige me to wait 
for some minutes before taking the magnetometer reading. The amount of this 
creeping is, however, very small compared with the static hysteresis. It occurs most 
conspicuously in the softest iron and at points near the beginning of the steep part of 
the magnetisation curve, and is much more marked in wires which are being magne¬ 
tised for the first time after annealing, than in wires which have been previously 
magnetised, and demagnetised by the method of reversals. The action goes on too 
long to be ascribable to the self-induction of the circuit, and indeed occurs most 
noticeably when the changes of the current are effected gradually by means of the 
slide; and I do not think that it can be regarded otherwise than as evidence of true 
magnetic viscosity. 
§ 53. Comparative Effect of Sudden and Gradual Changes of Magnetising Force , 
and Resultant Effects of Cyclic Changes of Magnetising Force .—It has long been 
well known from the experiments of Von Waltenhofen* and others that when a 
magnetising force is suddenly applied the induced magnetism is greater than when 
the force is gradually applied, and that when a magnetising force is suddenly 
withdrawn the residual magnetism is less than when the force is gradually withdrawn. 
My own experiments afford many confirmations of these facts, but they show that in 
very long uniformly magnetised rods the difference between the effects of sudden and 
the effects of gradual changes of magnetising force are not great. Thus, in the 
ballistic method, the difference in effect is not very material whether we produce a 
change of magnetising force in a series of numerous steps, or in a single step. The 
former process gives slightly, but very slightly, less total change of magnetism than 
the latter. And in the magnetometric method the magnetisation produced by raising 
the magnetising current continuously by means of a slide was found to differ but 
little from that given by a sudden establishment of the same current. The differences, 
* Po°'g\ Ann., 1863. 
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