420 
steel wire one millimetre in diameter instead 
of those found in the common form of mag- 
netometer ; and in the deflection experiment he 
uses the light mirror magnets which are found 
in Thomson’s reflecting-galvanometer. In 
simplicity and convenience, his plan certainly 
possesses many advantages, as well as in free- 
dom from certain errors which are likely to 
exist in the use of the more massive forms. 
It also has the merit of cheapness, enabling 
any one, at little expense, to determine this 
important element with a reasonable degree of 
accuracy. Neither method, however, is free 
from disadvantages, to which the author directs 
further attention in a note. 
This discussion is followed by a considera- 
tion of the methods of calculating the constants 
of a coil, and the construction of a standard 
galvanometer, the latter being described with 
such attention to details as to leave little to be 
desired. 
the use of the standard galvanometer in the 
graduation of other forms, selecting for this 
purpose Sir William Thomson’s potential and 
current galvanometers. ‘The construction of 
these instruments is described, and the process 
of graduation which is actually adopted in 
practice. While it is doubtless true that in- 
struments for the measure of current and elec- 
tromotive force which satisfy all the demands 
of the practical electrician have not yet been 
devised, Sir William Thomson’s unquestion- 
ably rank high among those at present in use. 
The most serious error which is likely to re- 
sult from their employment arises out of the 
change in the strength of field produced by 
the permanent magnets, which is pretty certain 
to occur. Mr. Gray suggests several methods 
of testing the field of the magnets which fur- 
nish valuable checks in their use. 
Two or three simple tests which are not re- 
ferred to will readily suggest themselves to any 
one making use of the instruments. 
The discussion of resistance-measurements, 
although not exhaustive, covers most of the 
ground; and especial attention is given to 
methods of measuring very low resistances, 
now a matter of greater importance than for- 
merly. A chapter is devoted to the measure- 
ment of the energy in electric circuits, which 
includes a valuable discussion of the theory of 
alternating-machines and methods especially | 
adapted to them. 
One of the most interesting features of the 
book is a description of several simple and 
ingenious methods of measuring intense mag- 
netic fields, suggested by Sir William Thom- 
son; and also the use of earth-inductors in 
SCIENCE. 
The author then proceeds to explain » 
[Von. II., No. 61. 
absolute measurements, as originally applied 
by Professor Rowland. , 
The closing chapter is devoted to a very 
satisfactory discussion of dimensional equa- 
tions. 
A list of errata accompanies the volume, and 
several errors not included therein are to be 
found ; but none are of great importance, or 
likely to mislead the reader; and altogether 
the book will be welcomed by every student 
of electricity. 
Mr. Murdock’s notes are intended to be 
supplementary to the Elementary lessons in 
electricity and magnetism by Silvanus P. 
Thompson, and consist, in the main, of am- 
plifications of some of the propositions in that 
work, with demonstrations in which a knowl- 
ledge of the elements of the calculus is assumed. 
Occasional extensions and additions are also 
made, which add much to the value of the book. 
It is likely to be of considerable use to the 
student of Professor Thompson’s elementary 
lessons, and it may also be used alone with 
little difficulty. Errors are here and there met 
with, the most notable of which are to be found 
in the definitions of units, originating either in 
gross carelessness, or in a confusion of ideas 
in the mind of the author. The distinction 
between work and rate of work, or activity as 
it is happily named by Sir William Thomson, 
is not regarded in the definitions. A coulomb 
ig defined as an ampere per second. A watt, 
which is activity, is defined as 10" ergs, which 
iswork. The watt and joule are declared iden- 
tical; although the first is activity, and the 
second is energy. The joule is defined as a 
quantity of heat: the suggestion of Sir W. 
Siemens was, that it should be a unit of work, 
equal to 10’ ergs. 
Thanks to the efforts of the British associa- 
tion and the international electrical congress, 
the nomenclature of electrical measurement is 
well-nigh perfect ; and it is both important and 
easy for the student to acquire in the begin- 
ning clear and accurate conceptions of the 
nature and relations of the units involved. 
Is there not, however, a tendency at the present 
time to overdo the matter of creating and nam- 
ing units? Too many will complicate rather 
than simplify processes of computation. The 
use of ‘ joule’ does not seem to be altogether 
free from criticism, on account of the fact that — 
the same name, or at least the initial J, has — 
long been in use as the symbol for the mechan- — 
ical equivalent of heat. 
pear to be any good reason for making a unit 
of the current which will evolve one cubic 
centimetre of mixed gases per minute, and 
There does not ap- 
