3G 
.MESSES. G E. C. SEAELE AND T G. BEDEOED 
in the ])resent paper, and, further, much of the general knowledge of magnetism needed 
in carrying out that research has been gained from his writings and from conversations 
with him. 
§ 3. It occurred to one of us some years ago that just as Ewixg had, in effect, 
applied Warbuhg’s theory to produce a practical hysteresis tester, so it might be 
possible to apply Hopkinson’s theory to the design of a method which should give 
aljsolute determinations of the energy dissipated through hysteresis as quicklv and 
as accurately as changes in magnetic induction are found by the aid of a ballistic 
galvanometer. It was evident that if this could be attained it would be possible to 
investigate the efiects of various phj^sical conditions—stress, temperature, the 
jjassage of an electric current, &c.—upon the hysteresis, with a comparatively very 
small expenditure of time. A preliminary account of the theory of the method 
was published in 1895,^ and since then much time has been spent in working out 
some details which make the method practical; as only a few weeks in each year 
have been available for tlie work, progress has been slow. 
In essentials the method is of course well known. In one of its forms it is in 
constant use among electrical engineers in testing by means of a watt-meter the 
energy dissipated in a transformer when its primary coil is traversed by an 
alternating current. In this case there is a steady deflexion of the watt-meter, 
and thus the watt-meter method is convenient in commercial work. 
From the scientific standpoint, the watt-meter method has the disadvantage that 
it is not possible to find the limits between which the magnetic induction alternates 
without the use of revolving contact-makers, or oscillographs, or other appliances. 
The “effective” voltage is indeed easily measured, Imt unless the w’ave-form of the 
curve of voltage is known, the limits of the induction cannot be found. 
With transformers of commercial dimensions the “ effective ” voltao-e is consider- 
able, Imt when the iron is reduced to a single wire only 1 or 2 sq. millims. in section 
the “ effective” voltage for 100 alternations per second does not exceed a few tenths 
of a volt, even when the secondary coil contains 1000 turns of Avire. We knoAv 
of no method by Avhich so small an alternating voltage can be measured Avith any 
accuracy. 
T1 le idea Avhich occurred to one of us in 1895 Avas to use a single reversal of the 
current to produce a “ tliroAv ” of a Ijallistic electro-dynamometer instead of an 
alternating current to produce a steady deflexion of a Avatt-meter. The present 
paper contains an account of the development of this idea and of its applications to 
mao'uetic research. 
o 
8o far as Ave knoAv, the ballistic method of measuring hysteresis is noA'el. In the 
endeavour to make it a practical method, Ave luiA’e met Avith many difficulties, and the 
main part of the Avork has been devoted to oA^ercoming those difficulties. 
* G. E. C. Searle, “A Method of Measuring the Loss of Energy in Hysteresis,” ‘ Proc. Camb. Phil. 
Soc.,’ A'ol. 9, Part I., NoA'cinber 11, 1895. 
