“LX Ge Magnetization of Iron in Bulk. 
By WY D1. THORNTON; DSe7 (Van) = 
$1. A method of testing the permeability of iron in bulk, with experi- 
| ments on large rings. 
-§ 2. The influence of core-currents upon the growth of the magnetizing 
| current. Comparison of coils with solid and laminated cores. 
§ 3, Calculation of subsidence or growth of current in coil with large | 
solid core of circular geehina. Evidence of sudden molecular 
movement as the iron approaches saturation. 
figs permeability of specimens of iron has long been 
known to depend upon their size, shape, and previous 
history. The usual method of determining magnetic pro- 
perties by tests on small samples cut or specially ‘cast from a 
larger mass cannot be entirely relied on to give the same— 
values that would be obtained if it were possible to apply 
tests to the material in bulk. It is especially necessary, in 
order to have uniform results, even from a batch of samples 
of the same metal, that they should have had the same | 
annealing or have been subjected to the same process and | 
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time of cooling. If they could be conveniently compared,.a . 
20 per cent. difference between the behaviour of a small_ 
specimen and of the actual magnet frame would be found to 
be not uncommon, and the permissible variation between’ 
castings of the same group is sometimes specified of this 
magnitude. 
One method of testing has been developed t free from most 
objecticns but not entir ely free, because the amount of metal 
examined is a very small portion of the whole. It seems, in 
fact, that accuracy cannot be guaranteed unless the specimens 
are large enough to be considered as “in bulk,” and are tested 
as a whole. 
The two methods at present in use with the smaller 
specimens are the ballistic and the magnetometric. The 
latter is only suitable for long thin rods, the tormer for cores 
which, either by reason of the low inductance or high re- 
sistance of their windings, have small time-constants, so that 
the total inductive change following any alteration of the 
exciting current is practically instantaneous. 
Some years ago the writer had occasion to measure the 
slow magnetic movements which occur on making or short- 
circuiting the exciting current of large dynamo-electric 
machines{. ‘To do this a recording millivoltmeter was con- 
nected to an exploring coil surrounding the core. Any 
* Communicated by the Physical | Society: read June 24, 1904. 
* Drysdale, Journal Inst. Elect. Engrs. vol. xxxi. p. 283. 
i Vide ‘The Electrical Engineer, vol. Xx1x, p. 525 ff. 
