[ 141 ] 
XVII. Notices respecting New Books. 
Absolute Measurements in Electricity and Magnetism. By ANDREW 
Gray, W.A., F.RS.E., Chief Assistant to the Professor of Natural 
Philosophy in the University of Glasgow [now Professor of Physics 
in University College, Bangor]. London: Macmillan and Co., 
1884 (pp. xiv+194; sm. 8vo). 
HE chief function of this little book is to describe the prin- 
ciples and methods of electrical measurement, as introduced 
and modified by Sir William Thomson, and in use at Glasgow 
University. With some slight additions it is a reprint of articles 
published in ‘ Nature’; and it contains in a convenient form some 
very useful practical details and hints, which are evidently given 
by one thoroughly conversant with the apparatus described. 
The first chapter contains an account of the usual method of 
determining the earth’s horizontal intensity and the moments of 
magnets, carried out with extemporized but quite satisfactory ap- 
pliances—applances, indeed, in many respects more in accord 
with the demands of theory than are the present Kew instruments, 
Then, after a chapter of definitions, comes the tangent galvanometer, 
with its elementary theory fully worked out. Then a full and 
instructive description of Sir W. Thomson’s two new “ graded ” 
galvanometers for measuring potential and current respectively. 
The range and versatilizy of these instruments render them very 
serviceable in a laboratory, though they are probably too complex 
and delicate for workshop manipulation ; and the account of their 
uses, by one who is so thoroughly familiar with their working as 
is Mr. Gray, gives the little book a unique value. 
Methods of measuring resistance, some of them well known, 
others less known, are given pretty fully ; and a very interesting 
chapter on the measurement of intense magnetic fields concludes 
the practical portion of the book. 
Not many electricians know how to measure the intensity of 
the field between the poles of an electromagnet, but it is a matter 
of vital importance in the construction of dynamos; and if the 
methods here described enable measurements of magnetic fields to 
be carried out with anything like the same glibness as is now the 
case with currents and resistances, the book will have done good 
service and much aided the practical development of the science. 
This is indeed too much to expect, but a beginning can now be 
made; and the still-needed practical instrument, convenient and 
simple in use, yet able to effect measurement of fields with 
reasonable accuracy, will before long be invented. 
Concerning the more theoretical portions of the book, they are 
not different from what is already common property ; nevertheless, 
isolated as they here are from more difficult matter, and embodied 
in a small compass, these portions will be found very useful for 
college classes such as ordinarily go by the name of “ Senior.” 
