JUNE 5, 1902] 
INA TORE 
125 
published in the Zvectyician or communicated to scientific 
societies. But in one sense the book may be said to be 
entirely new, as it presents for the first time the results 
of Mrs. Ayrton’s work in the form of a connected whole 
in which the interdependence of the various parts is 
made manifest. 
After a short chapter on the general appearance of the 
arc, Mrs. Ayrton gives, in a chapter of nearly eighty 
pages, a history of its discovery and development and of 
the investigations to which it has been subjected. 
Though the discovery is usually attributed to Davy, it is 
clear from the quotations given by Mrs. Ayrton that the 
arc was evolved from the electric spark without any 
distinct recognition of the difference between the two ; to 
Davy, however, belongs the credit of the first description 
of an undoubted arc, from which dates, probably, its 
recognition as a distinct phenomenon. The remainder 
of the chapter, in which all the important papers on the 
are are considered, is of great value, especially, perhaps, 
to the student, as it leads by easy gradation from the 
simpler theories which were advanced at first to the more 
complex and complete explanations which have to be put 
forward now that all the problems arising are more 
fully realised. 
Mrs. Ayrton then passes on to the behaviour of the 
arc immediately after striking, and to the necessity of 
working with “normal” arcs, that is, arcs in which the 
P.D. has assumed its steady value for the given current 
and length of arc. The laws that govern the P.D., 
current and other electrical quantities are then considered 
in detail, as a result of which Mrs. Ayrton’s now familiar 
equation connecting the P.D. with the current and 
length of arc is derived. The equation, which is of the 
form V=a+ él + © where a, 4, c and @ are con- 
stants depending on the carbons and V, / and A the 
P.D., length of arc and current respectively, is shown to 
fit in with the results of the experiments of previous 
investigators, even though the equations which they had 
themselves advanced were different or less complete. 
This equation, which applies only to solid carbons, is 
very accurate over the range studied by Mrs. Ayrton. 
The {effect of cores in cored carbons is considered, and 
it is shown, by an examination of the P.D. between the 
carbons and the arc, how the various terms of the equa- 
tion can be correlated with the physical phenomena in 
the arc. 
Especial interest attaches to the above equation on 
account of its connection with the theory advanced in 
the final chapter. But apart from this, the whole of this 
portion of the work is of the highest intrinsic value. We 
cannot, with the space at our disposal, consider it in 
any detail, but we might particularly draw attention to 
chapter vili., which deals with the stability of the arc, 
and to chapter x., on hissing arcs. The chapters dealing 
with the efficiency of the arc, considered first as a con- 
sumer of power and secondly as a source of light, should 
appeal strongly to those who are interested in arcs from 
the practical standpoint, especially as they show how 
much the commercial standard falls below that actually 
obtainable. The arc enjoys at present the distinction of 
being the cheapest form of artificial light, so that the only 
NO. 1701, VOL. 66] 
competition to be met is that between different forms of 
lamps ; but this is quite sufficient to lead to the ad- 
visability of an attempt at ‘morovement by the scientific 
study of the best conditions of working. 
In chapter xii., Mrs. Ayrton puts forward her theory of 
the arc. It is hardly fair to consider this theory apart 
from the rest of the book, as it arises naturally out of the 
views advanced therein. For a long time experimenters 
have been divided into two camps, those who believed 
in the existence of a back E.M.F. in the arc and those 
who did not. According to Mrs. Ayrton’s view, it is not 
necessary to assume the existence of a back E.M.F. to 
account for the high fall of potential between the positive 
carbon and the arc, as it can be explained in another 
way. The conducting part of the arc consists, on this 
view, of a very thin film of true carbon vapour at the 
crater and of carbon mist throughout the rest of the arc. 
The very high specific resistance of the carbon vapour 
accounts, not only for the fall of P.D., but also for the 
volatilisation and cratering of the positive carbon. It is 
shown from actual measurements of the cross-sections of 
the mist and vapour that their variation with current and 
length of arc lead to a law of variation of P.D. precisely 
similar to that given in the above equation. If we may 
venture criticism, we should like to point out that the 
explanation suffers from being only qualitative, and 
though the form of the equation is the same there is 
nothing to show that the constants are of the same 
magnitude. It remains to be shown by experiment 
whether this is the case or not. 
The method of measuring the “true” resistance of 
the arc by a superimposed alternating current fails, as 
Mrs. Ayrton shows, unless it can be proved that the 
added current in no way affects either the resistance or 
the back E.M.F. (if existent). It is well known that this 
method leads in some cases to negative results, and 
Mrs. Ayrton points out that it may lead to any value 
from a large negative one to that of the “true” resistance, 
according to the frequency of the added current, which 
must be as high as “‘ many thousands of alternations per 
second for the resistance of the arc not to be altered by it.” 
In a recent paper by Mr. Duddell, read before the Royal 
Society, the results of experiments with a frequency as 
high as 120,000 alternations per second were given, and 
the author claimed that they showed a true resistance of 
about four ohms and a back E.M.F. of twelve volts. 
Unfortunately, this paper was not published at the time 
of the completion of Mrs. Ayrton’s book, but it would be 
interesting to learn whether she can harmonise it with 
her theory. Incidentally, it may be noted that Mrs. 
Ayrton’s explanation accounts for the effects produced 
by cores and for the shapes to which the carbons burn, 
and there can be little doubt that, whether or not it 
proves sufficient to rank as a complete theory, it is a 
great advance towards the clear appreciation and the 
solution of the problems presented. 
In conclusion, we may commend the excellent draw- 
ings of arcs and carbons with which, in addition to 
curves and diagrams, the book is copiously illustrated. 
Altogether the book deserves to rank as one of the most 
important contributions to electrical literature that has 
appeared of late years. M. S. 
