NATURE 



559 



THURSDAY, JANUARY 20,. 1916. 



ARC LIGHT. 

 Electric Arc Phenomena. By Ewald Rasch. 

 Translated from the German by K. Tornberg. 

 xvi + 194 pp. (London: Constable and Co., 

 Ltd., 191 5.) Price 8s. 6d. net. 



THIS book surprises the reader by its extra- 

 ordinary inequality : its excellencies in 

 -ome directions and its absolute badness in 

 others. Either the author, who is understood to 

 l)e a professor at Potsdam, is amazing-ly ignorant 

 of scientific and technical progress outside Ger- 

 many, or else he suppresses the work achieved 

 by discoverers in other nations. And his trans- 

 lator, who is apparently American, makes no 

 attempt to remedy the very obvious limitations 

 ot the author : he not even attempts to reclaim 

 for American pioneers in arc lighting — Elihu 

 Thomson, Brush, Steinmetz, and more recent 

 workers — the credit which is justly theirs. As 

 for English investigators of the electric arc, they 

 are almost all ignored except Mrs. Ayrton, whose 

 work the author does not altogether understand, 

 and Prof. Silvanus Thompson, to whom he 

 gives more credit than that industrious person 

 ever claimed. His historical methods are pecu- 

 liar. For example, when referring to Davy's 

 early accounts of his experiments on taking spark 

 discharges from a voltaic pile between electrodes 

 of charcoal — experiments which certainly did not 

 begin before the invention of the pile in 1800 — 

 he quotes a passage in the first person from 

 Priestley's "History of Electricity," so making 

 Priestley, whose last edition was in 1794, "respon- 

 sible for Davy's words. Again, referring to the 

 commercial impossibility of applying the arc for 

 electric lighting so long as a battery of voltaic cells 

 was the only available source, the author, oblivious 

 of Holmes's success in establishing electric lights 

 in lighthouses by means of currents generated 

 by an alternator, oblivious, too, of Faraday's 

 report and lecture thereon in 1862, makes the 

 following absurd statement : — 



'• But it was not till seventy years later " [than 

 Volta's discovery of 1800] "that the electric arc 

 could become of practical importance as a source 

 of light, when, thanks to the invention of the 

 dynamo by Werner von Siemens, Hefner Alteneck, 

 and others, a convenient and economical source 

 was provided, which substituted electrodynamic 

 action for the electrochemical action of galvanic 

 primary cells." 



It is true that in 1867 Werner von Siemens in- 

 vented a particular form of dynamo, and that in 

 1873 Hefner Alteneck introduced modifications of 

 NO. 2412. VOL. 96] 



the winding of armatures. These improvements 

 scarcely justify such sweeping claims in their 

 favoiir, or the ignoring of Wilde, Holmes, and 

 Gramme. Again, it is foolish to claim for Davy, 

 as the author does on p. 4, the employment of " a 

 series resistance," and "the selection of a suitable 

 voltage " to produce stable flame discharges. 

 Some of the alleged facts as to arcs cause us to 

 rub our eyes and wonder whether the author, who 

 again and again quotes himself as having taken 

 out various patents on arc lighting, has ever veri- 

 fied them. For instance, he alleges that heat 

 conduction along the carbons is responsible for a 

 waste of 79 per cent, of the energy supplied to 

 the arc. Also he asserts (on the authority of 

 Arons) the impossibility of maintaining an alter- 

 nating arc between metallic electrodes on account 

 of their great thermal conductivity — an assertion 

 abundantly contradicted by the researches of 

 Pfund, and arc-furnace experience. The author 

 seems to think that the starting of an arc by any 

 other process than bringing the tips of the carbons 

 into contact and then parting them was unknown 

 until he discovered other ways in 1899; and that 

 these other ways are of an importance comparable 

 with the usual contact method. His investiga- 

 tions on the possible use of kathodes consisting of 

 hot metallic oxides and other solid electrolytes 

 are doubtless a useful contribution to knowledge : 

 but they scarcely justify his attitude towards 

 Wehnelt. 



On p. 1 1 the author declares that a stable arc 

 is possible, and can be maintained, only when the 

 kathode surface has a sufficiently high tempera- 

 ture : yet on p. 163 we discover that even com- 

 paratively cold liquids, aqueous solutions, and 

 fused salts are excellent kathodes, and are capable 

 of producing surprisingly beautiful arcs. His 

 definition of an arc is as follows : — " Broadly 

 speaking, one must designate as an electric arc 

 any. continuous discharge occurring between elec- 

 trodes of different potential and serving as a 

 source of light, where at least one of the elec- 

 trodes — the kathode — is kept at a high tempera- 

 ture either by the current passing or by auxiliary 

 means." According to this definition a Duddell 

 arc which serves, not as a source of light, but as 

 a generator of oscillations, is not an arc. The 

 author states that the anode crater acts, "in a 

 sense," as a reflector upon which the kathode 

 radiates its heat. Since the crater anode is 

 hotter, and emits twenty times as much light as 

 the tip of the kathode, its action as a reflector 

 can only be admitted in a very special sense 

 indeed. 



The instructions which the author gives on 

 pp. 19-21 for adjustment of the arc seem to miss 



Y 



