II. Propagation of Magnetization of Iron as affected by the Electric Currents 
in the Iron. 
By J. Horxryson, F.R.S., and E. Witson.* 
Received May 17,—Read May 31, 1894. 
Part I. 
Ir is not unfamiliar to those who have worked on large dynamos with the ballistic 
galvanometer, that the indications of the galvanometer do not give the whole changes 
which occur in the induction. Let the deflections of the galvanometer connected to 
an exploring coil be observed when the main current in the magnetic coils is reversed. 
The first elongation will be much greater than the second in the other direction, and 
probably the third greater than the second—showing that a continued current exists 
in one direction for a time comparable with the time of oscillation of the galvanometer. 
These effects cannot be got rid of, though they can be diminished by passing the 
exciting current through a non-inductive resistance and increasing the electromotive 
force employed. This if carried far enough would be effective if the iron of the cores 
were divided so that no currents could exist in the iron; but the currents in the iron, 
if the core is solid, continue for a considerable time and maintain the magnetism of 
the interior of the core in the direction it had before reversal of current. It was one 
of our objects to investigate this more closely by ascertaining the changes occurring at 
different depths in a core in terms of the time after reversal has been made. 
The experiments were carried out in the Siemens Laboratory, King’s College, 
London ; and the electro-magnet used is shown in fig. 1. It consists in its first form, 
the results of which though instructive are not satisfactory, of two vertical wrought- 
iron cores, 18 inches long and 4 inches diameter, wound with 2595 and 2613 turns 
respectively of No. 16 B.W.G. cotton-covered copper wire—the resistance of the two 
coils in series being 16°3 ohms. The yoke is of wrought-iron 4 inches square in 
section and 2 feet long. The pole-pieces are of wrought-iron 4 inches square, and all 
surfaces in contact are truly planed. One of the pole-pieces is turned down at the 
end, which butts on the other pole-piece, for half an inch of its length to a diameter 
of 4 inches; and three circular grooves are cut in the abutting face having mean 
* The experimental work of this paper was in part carried out by three of the Student Demonstrators 
of the Sizmexs Laboratory, King’s College, London, Messrs. Brazit, Arcuison, and Greennam. We 
wish to express our thanks to them for their zealous co-operation. 
19.2.90 
