PROPAGATION OF MAGNETIZATION OF IRON. 101 
terminal T, to 9 convolutions round the centre of the tube again along the groove to 
terminal T,. Nine convolutions were also wound round the outside tube Ag, the ends 
of which are connected to the terminals T;, T, respectively. 
The tubes and rod were made by Sir J. WuitwortH and Co., of Manchester, 
and a considerable force was required to drive the pieces into their proper position. 
Our best thanks are due to Professor KENNEDY and his assistants for the putting 
together of these pieces by means of a 50-ton hydraulic testing machine. We are 
aware that the surfaces are somewhat scored by the hydraulic pressure, and the 
magnetic qualities may be slightly different for layers of the soft steel near these 
surfaces, but they serve just as well for the purpose of our experiments. 
Fig. 8. 




Systematic experiments were then commenced. The magnetizing coils on the 
magnets were placed in parallel with one another, and a total current of 1°75 amperes 
(that is, °87 ampere in each coil), due to 5 storage cells, was reversed through the 
coils. The arrangement of apparatus is shown in fig. 3, except that the pole-pieces 
are replaced by the soft steel tubes shown in fig. 8, and the non-inductive resistance 
C is removed. We have now three exploring coils instead of four, and these are 
marked 1, 2, 3 respectively, starting with the coil of smallest diameter. For the 
purpose of obtaining the current curve, the D’Arsonval was placed across a non- 
inductive resistance of $ ohm in the circuit of the magnetizing coils. Fig. 9 gives 
a set of curves obtained with the 5 cells, and also another set obtained by a reversal 
of 1°8 amperes given by 54 cells—a non-inductive resistance being placed in the 
circuit to adjust the current. 
The effect of reversing the same maximum current with two different potentials is 
very marked. Take coil No. 1. With 5 cells the maximum rate of change of 
induction occurs at 9 seconds after reversal, at which epoch the current in the copper 
coils is about 1 ampere, the maximum current being 1°75. With 54 cells the 
maximum rate of change of induction occurs at 4 seconds, and here the current in the 
copper coils is nearly a maximum. We therefore chose to work with 54 cells, thus 
avoiding a magnetizing force due to the current in the copper coils varying a 
considerable times after reversal. 
