144 Mr. A. Gray on the Determination of the 
I may refer to my paper in the Philosophical Magazine for 
September 1871 for some investigations which confirm Mr. 
Ferrel's results. 
Bushmills, co. Antrim, 
July 20, 1883. 
XXII. On the Determination in Absolute Units of the Inten- 
sities of Powerful Magnetic Fields. By A. GrRAT, M.A., 
F.R.S.E., Chief Assistant to the Professor of Natural 
Philosophy in the University of Glasgow*. 
SO far as I know, no method has yet been published by 
which the intensity of a powerful magnetic field, such as 
that in the space in which the armature-coils of a dynamo 
move between the poles of the field-magnets, can be deter- 
mined in absolute measure ; and inventors of dynamos and 
other experimenters in practical electricity have hitherto, at 
a great expense of time and money, had to construct their 
actual apparatus, and find by the results obtained in actual 
practice the efficiency or non-efficiency of their arrangements. 
So much so has this been the case that it has even been 
held that the absolute value of the intensity of a mag- 
netic field is a thing with which practical electricians have 
no concern, that, although theoretically it may have some 
importance, it has no application in practice. That all 
measurements, whether of electric or of magnetic quanti- 
ties, should be expressed in absolute units — that is, in units 
altogether independent of the locality, surroundings, and 
apparatus of the experimenter — is, I think, of very great im- 
portance, if these results are to be of any service to others. 
And it would, 1 feel sure, be a great benefit to all engaged in 
practical electrical work, if the intensities of the magnetic 
fields obtainable with various forms of electromagnets, made 
with different kinds of iron, were determined in absolute 
units and published with full particulars of the apparatus. 
But failing the knowledge which would be derived from ex- 
periments such as these, a very great saving of time and 
money might be made if the inventor of a dynamo, for 
example, were to first make models of his magnets on a small 
scale, and determine the magnetic-field intensities obtainable 
in them with different current-strengths, and then to reason 
from the results in the model to those in the full-sizod machine. 
The object of the present paper is to describe some methods 
by which the intensity of a powerful magnetic field may be 
determined in absolute units. The methods are wholly due 
to Sir William Thomson ; but for the sketch of theory given 
* Communicated by the Author. 
