December 20, 1900] 



NA TURE 



I-83. 



rendering it intermittent by blowing it with a magnet. 

 This method did not answer, as the intermittence was too 

 irregular ; and in order to try and overcome this irregu- 

 larity Mr. Duddell shunted the arc with a condenser, and 

 found that the arc immediately became intermittent with- 

 out any blowing, and emitted a musical note. It appeared 

 that the leads from the arc to the condenser possessed 

 appreciable self-induction, and that if this were destroyed 

 the musical note ceased. It thus became evident that a 

 direct current arc between solid carbons, when shunted 

 by a capacity in series with self-induction, supplied alter- 

 nating current to the shunt circuit— the complete circuit 

 consisting of the arc, self-induction, and capacity in 

 series, the arc thus acting as a converter of direct into 

 alternating current energy. 



This eflfect can only be produced when the arc has the 

 ratio of a small change in P.D. (aV)to the corresponding 

 change in current (9A) negative; and when this ratio 

 aV/9A is numerically greater than r, the resistance of 

 the condenser circuit. This was proved by Mr. Duddell 

 with two experiments. With a cored carbon arc for which 

 9V/9A is positive he showed it was impossible to obtain 

 a musical note. And using a solid carbon arc shunted 

 by a condenser and self-induction and giving out a clear 

 note, he showed that by increasing the resistance of the 

 condenser circuit the sound steadily diminished and 

 finally completely died out when this resistance became 

 numerically equal to 9V/9A. Any cause tending to 

 dissipate the energy in the condenser circuit, such as, for 

 example, the hysteresis of an iron wire core introduced 

 into the self-induction, or any complete circuit, such as a 

 sheet of iron or a closed ring of wire, brought near it, 

 will also stop the note. This phenomenon suggests, as 

 was experimentally demonstrated, a very simple and 

 valuable method of obtaining oscillating currents of 

 any desired frequency for experiments on magnetic 

 space telegraphy. 



Some experiments with metal arcs brought out two 

 points of great practical importance. Mr. Duddell found 

 that on shunting an arc between metal electrodes by a 

 condenser the arc went out. The high rise of P.D. 

 caused by thus suddenly breaking an inductive arc 

 circuit may be sufficiently great to break down the 

 msulation of the leads, as was shown by an experiment, 

 in which a weak place in the insulation was introduced by 

 bringing the two conductors to brass plates separated by 

 a sheet of paper : every time the arc was shunted and 

 put out, the paper was pierced by a spark. The same 

 result was obtained by connecting the condenser per- 

 manently across the arc terminals and trying to strike the 

 arc. This has important bearing on the practical use of 

 metal switches, since it shows that the arcing at breakmg 

 should be encouraged rather than suppressed, smce 

 if there be capacity as a shunt to the switch-con- 

 tacts and self-induction in the main circuit, a high 

 rise in P.D. will occur, and may cause serious 

 damage to the leads. As another instance of 'the prac- 

 tical application of this effect, Mr. Duddell showed that, 

 when using an induction coil, a far longer spark could be 

 obtained if the connections were made so that the con- 

 tact maker first broke the circuit and then shunted a 

 condenser across the gap to blow out the spark, instead 

 of, as has always hitherto been done, having the gap 

 permanently shunted by a condenser. 



Mr. Duddell concluded his paper by showing that the 

 note emitted by a musical arc could be tuned by adjust- 

 ing the self-induction and capacity in the shunt circuit. 

 A keyboard was arranged which shunted different capaci- 

 ties and self-inductions across the arc, and by this means 

 two complete octaves were obtainable. Four arcs were 

 arranged in series to increase the loudness of the sound, 

 and a very distinct and not unmusical rendering of "God 

 Save the Queen " was played on them. 

 The Central Technical College may well be congratu- 



NO. 1625. VOL. 63I 



lated on the work on the arc that has been done in its- 

 laboratories. Within the last two years there have been 

 four most important papers on this subject read before- 

 the Institution of Electrical Engineers— Messrs. Duddell 

 and Marchant's paper on the Alternate-current Arc,. 

 Mrs. Ayrton's paper on "Hissing Arcs," her paper at 

 the Paris Congress on the "Light given put by the 

 Direct Current Arc," and the paper by Mr. Duddell 

 above described— all emanating from the College, and 

 each contributing in no small degree to the elucidation 

 of the many very difficult problems which the arc 

 presents. 



A BIRD-BOOK FOR YOUNG PEOPLED 



W'HETHER designedly or no, this attractive little- 

 volume is fortunate in the time of its appearance, 

 since it forms an appropriate Christmas gift to young 

 persons of both sexes interested in observing the ways 

 of the birds of their own neighbourhood. And it is not 

 even necessary that such young people should be resi- 

 dent in the country to appreciate the book, for the author, 

 as in his account of the gulls on the Thames in wmter,. 

 shows that there is much to be learnt with regard to 

 bird-life even by the dweller in the metropolis. The. 

 appearance of a bird -book of this nature at the Christmas 

 season is also appropriate in that it tends to draw atten- 

 tion to the severe hardships our feathered friends have 

 frequently to suffer at this time of year, and thus attracts 

 sympathy and attention to their wants. 



To those of our readers who are familiar with the 

 Messrs. Kearton by their previous works, no recommend- 

 ation will be necessary in the case of the volume before 

 us ; while to those who have yet to become acquamted 

 with the earlier literary and artistic eff'orts of these 

 gentlemen, their new production will come as a welcome 

 surprise. For although primarily intended for young 

 people, it must not for a moment be supposed that the- 

 author's latest volume is not calculated to interest readers 

 of more mature years. Indeed, the beauty and attrac- 

 tive character of the illustrations (two of which, by the 

 courtesy of the publishers, we are enabled to reproduce) 

 are alone quite sufficient to render the volume accept- 

 able to readers of all classes and all ages. Mr. C. 

 Kearton seems, indeed, almost to have surpassed him- 

 self, not only in the execution of the photographs, but 

 in the interesting phases of bird-life and bird-architecture 

 he has portrayed. All the photographs, it appears, have 

 been specially taken for this particular volume, and as 

 they reach one hundred in number, while their venue- 

 extends from the Thames Embankment to the Hebrides, 

 some idea may be gathered of the amount of time, labour 

 and money expended in its production. 



A feature of the book is the attention devoted to nests,, 

 eggs and young birds ; and although the style is essen- 

 tially popular and suited to the capacity of the readers 

 for whom it is primarily intended, older ornithologists 

 will scarcely fail to be interested in the chapters on these 

 subjects. In particular we may draw attention to the 

 eight photographs on p. 99, the first of which represents 

 a blackbird's egg on the day previous to hatching, and 

 the other seven the young bird from day to day. By 

 a careful arrangement and adjustment of the camera, the 

 young bird was photographed to the same scale, and the 

 marvellous rapidity of its development— especially be- 

 tween the fourth and seventh day of its existence— will 

 come almost as a revelation to many readers. Unfor- 

 tunately the further progress of the daily portraiture was 

 brought to an abrupt termination by the unwelcome at- 

 tentions of a cat. The subject is, however, full of promise, 

 and one worthy to be taken up by other photographers*. 



1 " Our Bird Friends : a Book for all Boys and Girls." By R. Kearton. 

 With photographic illustrations by C. Kearton. Pp. xvi + 215- Illustrated. 

 (London : Cassell and Co., Ltd., 1900.) 



