530 
PROFESSOR J. A. EWING ON EXPERIMENTAL 
cells, through a solenoid of moderately fine wire wound closely round the rod or ring. 
Even when the solenoid consisted of more than one layer, as it frequently did, the 
correction for the air space in evaluating 33 was negligible, unless was unusually 
high. The force was varied, in the ballistic observations, by having in circuit with 
the magnetising solenoid a set of resistance coils which could be plugged up or 
inserted step by step so as to give a series of sudden increments or decrements in the 
value of <§. The magnetising current was measured by another mirror galvanometer, 
very strongly shunted, and its value in absolute measure was determined by seeing 
what deflection a given number of gravity cells in good condition gave through a 
given total resistance. In reducing the observations I have assumed that a gravity 
Daniell cell in good condition, when put in circuit with a total resistance of n 
British Association units gives a current of — amperes. This agrees well with 
Dr. C. R. A. Weight’s recent determination;'" and an experiment of my own, in 
which the current from one cell, was adjusted by a variable resistance until it just 
sufficed to neutralise, in a vertical solenoid, the vertical component of the earth’s 
force, gave a closely accordant value. 
To have aimed at any very high degree of exactness in the reduction of magnetisa¬ 
tion and magnetising force to absolute measure would have been a waste of time. 
The magnetic differences between different specimens of iron are so great that no 
useful purpose is served in determining absolutely the magnetic quality of any one 
specimen with extreme precision. On the other hand, even a somewhat rough 
reduction to absolute measure is so exceedingly serviceable in making results intelli¬ 
gible and definite in themselves, as well as capable of comparison with the work of 
other observers, as to justify fully the very considerable amount of arithmetical 
labour it involves. In dealing with measurements in which comparison of results 
was to be made, as, for instance, in the magnetisation of the same piece in different 
states of stress, I have been careful to make the reduced values accurately comparable 
with each other. 
§ 10. Magnetisation of an Iron Ring .—In the following experiment a welded ring 
of moderately soft iron wire was subjected to a magnetising force, which was applied 
by a series of sudden steps up to 9 ‘14 c.g.s. units, then reduced by steps to 0, and 
then reapplied, also by steps—with the view of testing the question raised in § 2, 
as to whether hysteresis occurs in the relation of 33 to <§ during the removal and 
reapplication of magnetising force, In this, the first example to be quoted, it may he 
well to give the numerical data and to exhibit the reduction in full: in subsequent 
examples a graphic representation of the reduced results, with occasionally a numerical 
statement of them, will suffice. 
* Phil. Mag. xiii. (1882), p. 265. 
