Properties of an Extensive Series of Alloys of Lion. 17 
rod of aluminium-iron was drawn down to a long wire till its length became 
452 diameters. Even at this, after careful annealing, the most gentle reversal 
of the wire almost completely reversed its magnetic state. Nothing could more 
strikingly illustrate the wonderful magnetic softness of this alloy, inasmuch 
as Professor Ewing has shown* that, when the length of a very soft iron rod 
is 400 to 500 diameters, the demagnetising reaction of the ends became negligible. 
That is to say, soft iron retains its magnetic state, like a ring of iron, when the 
magnetising force is removed: in our experiment, it is true, the action of the 
feeble magnetising force of the Earth is not removed from the rod, but reversed. 
Fic. 7.—Permeability curves obtained from slender rods een the ballistic method. 
It was convenient to use the vertical force of the Earth’s field as the first step 
in the series of magnetising forces to which the rods were subjected in the 
experiments on permeability plotted in figures 5 and 6.t In the ballistic 
experiments next to be described, the rods could be placed horizontally, east-and- 
west, so that no step in the magnetisation was due to the Earth. 
Both the long slender rods, S.C. I. and aluminium-iron, of 260 diameters in 
length, were tested ballistically by means of an excellent Crompton ballistic 
galvanometer. For this purpose a small secondary coil about 3:5 em. long was 
wound on a thin copper sheath which could be slipped fairly tightly on to 
* Magnetic Induction in Iron and other Metals.’ New edition, page 34. 
{| In determining the cyclic curves to be shown later on, it was, of course, necessary to neutralize the 
magnetisation due to the Earth by a compensating current around the solenoid. 
TRANS. ROY. DUB. SOC., N.S., VOL. VIII., PART I. D 
