RESEARCHES IN MAGNETISM. 
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advantage which a ring-shaped core, or core forming a complete magnetic circuit, 
possesses over a short bar core with ends. In an ordinary induction coil, so long as 
the current in the primary circuit is merely made and broken, a short core is 
necessary, since a ring core would lose but a small percentage of its magnetism at 
each break ; but where reversal of the magnetising current takes place a core 
approximating to the condition of endlessness has an advantage, in respect of power, 
which fig. 3 makes obvious.] 
§ 35. Heating Effect of Lower Cycles .—In most of the examples cited above (§ 33), 
in which the energy expended in cyclic changes of magnetisation has been determined, 
the magnetisation was strong, lying in the region of so-called saturation. It seemed 
desirable to measure the energy expended in carrying iron and steel through double 
reversals of weaker magnetisation, and accordingly the two following special experi¬ 
ments were made. 
In the first a piece of annealed iron wire, 0 - 0.78 centim. in diameter and 29 centims. 
long, was tested by the direct magnetometric method. Starting initially with a state 
of no magnetisation, the magnetising force was slowly raised to 1‘5, then slowly 
reversed to —1‘5, and then slowly re-reversed to 1‘5, numerous intermediate observa¬ 
tions of 3 an d >§ being made. The curves showing the relation of 3 to <§ in this 
process were afterwards plotted, and the area enclosed by them was measured. Then 
the magnetising force was further increased to I’95, and another cycle performed by 
reversing and re-reversing it. Next it was raised to 2’56, and another cycle performed 
by double reversal. Next to 3’01, when another cycle was performed, and so on, until 
the last double reversal took place between the values + 75 '2 and —75'2 for <§. In 
this way 10 areas were measured, giving 10 values of the energy expended in reversing 
and re-reversing the magnetisation of the wire at different grades of intensity. The 
whole process is shown graphically in Plate 60, fig. 15, which is a very much reduced 
version of the diagram by help of which the areas were measured. It would take too 
much space to reproduce here all the readings taken in determining these curves, as no 
fewer than 338 corresponding values of the magnetisation and the magnetising field 
were measured. The following are the values of 3, 33, and >§ at the successive points 
at which the magnetism was reversed in sign, and the corresponding values in ergs 
of — for the operation of double reversal. The last column gives the rise of 
temperature caused by one complete operation of double reversal of magnetism. 
