PROPAGATION OF MAGNETIZATION OF IRON. 109 
their area, and their lengths are alike as 1:n; the inductions through them, when 
the inductions per centimetre are the same, are as the areas, that is, as 1:n*. Hence 
if the inductions change at rates inversely proportional to 1: n’, the currents between 
corresponding radii will be the same at times in the ratio of 1: n”, and the magnetizing 
forces will also be the same. 
Magnets of sixteen inches diameter are not uncommon ; with such a magnet, the 
magnetizing force being 37 and the magnetizing current being compelled to at once 
attain its full value, it will take over a minute for the centre of the iron to attain its 
full inductive value. 
On the other hand, with a wire or bundle of wires, each 1 millim. diameter, and 
a magnetizing force between 3 and 5, which gives the longest times with our bar, 
the centre of the wire will be experiencing its greatest rate of change in about 
soo second. This is a magnetizing force similar to those used in transformers, and 
naturally leads us to the second part of our experiments. 
Part I].—ALTERNATE CURRENTS. 
This part of the subject has a practical bearing in the case of alternate current 
transformer cores, and the armature cores of dynamo-electric machines. 
The alternate currents used have periodic times, varying from 4 to 80 seconds, and 
were obtained from a battery of 54 storage cells by means of a liquid reverser,* shown 
in elevation and plan in figs. 14 and 15. It consists of two upright curved plates of 
sheet copper, AA, between which were rotated two similar plates, BB, connected 
with collecting rings, DD, from which the current was led away by brushes to the 
primary circuit of the magnet. The copper plates are placed in a weak solution of 
copper sulphate in a porcelain jar. The inner copper plates, and the collecting rings, 
are fixed to a vertical shaft, S, which can be rotated at any desired speed by means 
ot the gearing shown in the figure. The outer plates are connected to the terminals 
of the battery of storage cells, and the arrangement gives approximately a sine curve 
of current when working through a non-inductive resistance. 
The experiments were made with the same electro-magnet and Whitworth steel 
tubes described in Part I. of this paper. Fig. 16 gives a diagram of connections in 
which M is the current reverser, G is the Thomson graded current meter for 
measuring the maximum current in the copper coils, and W is the electro-magnet. A 
small, non-inductive resistance, placed in the primary circuit served to give the curve 
of current by observations on the D’Arsonval galvanometer, D, of the time variation 
of the potential difference between its ends. The D’Arsonval galvanometer was also 
used, as in Part I., for observing the electromotive forces of the exploring coils 1, 2, 
and 3 (see fig. 8, Part 1.), R being an adjustable resistance in its circuit for the 
purpose of keeping the deflections on the scale. 
* This form of reverser is due to Professor Ewinq. 
