l82 



NATURE 



[December 20, 1900 



■their decision their photograph of the sternum, which is 

 typically Dromaean, alone gives ample support. 



Other remarkable features are the slenderness of the 

 tarso-metatarsus and lower portion of the tibio-tarsus, 

 and the general feebleness of the digits, the ungual 

 phalanges of which are small, and believed to have borne 

 ^'flattened nails rather than sharp and powerful claws, 

 which could have been of little service for scratching 

 purposes," the whole pedal skeleton, in fact, being in 

 striking contrast with the massive proportions of the 

 upper-leg bones and sternum. There is a moderate 

 fibula well preserved. 



Two small fragments of the coraco- scapula and some 

 three or four ribs are described. Of the carpus there is 

 no trace, and doubt besets a small bone referred to 

 as a possible phalanx of the fore-limb. Concerning the 

 anti-brachium, however, the radius and a possible ulna 

 are preserved ; and for the former the authors give 

 measurements which show that, in contradistinction to 

 that of all other Ratitce, it far exceeds the humerus in 

 length — a feature in respect to which the Emeu comes 

 most nearly approximate but is still a long way behind. 



Of remains in good preservation, or that, by the in- 

 genious method of preparation adopted, upon which we 

 have already commented (Nature, vol. Ixi, p. 276), could 

 be rendered serviceable, those of the tibio-tarsalia were 

 by far the most numerous ; and in the present memoir 

 ithe authors devote special attention to chemical action 

 brought to bear upon those bones found nearest the sur- 

 face, to which is due their friability and peculiar texture, 

 associated with the formation of crystals, mostly of halite, 

 admixed with gypsum, glauberite and alunite, by which 

 they had become impregnated. The Memoir gives 

 promise of further interesting results, and any one at 

 home desirous of examining the remains will now find in 

 the Geological Department of our National Museum at 

 South Kensington a fine example of a hind-limb, in which 

 the extraordinary diversity in bulk of the opposite ends 

 of the tibio-tarsus, and the still more noteworthy slen- 

 derness of the innermost digit, must be seen to be 

 appreciated. • 



SOME EXPERIMENTS ON THE DIRECT- 

 CURRENT ARC. 



ON Thursday last, December 13, 'Mr. W. Duddell 

 read before the Institution of Electrical Engineers 

 a paper on " Rapid Variations in the Current through the 

 Direct-Current Arc," which he illustrated by experi- 

 ments. Members of the Institution have already learnt 

 from the experimental demonstration given by Mr. 

 Duddell in 1898, when he read the paper by Dr. 

 Marchant and himself on the alternate current arc, to 

 expect from him most interesting experiments. Nor 

 were they disappointed last Thursday. It is perhaps too 

 much to say that the experiments then shown excelled in 

 beauty and interest those exhibited on the former occa- 

 sion, but they fully maintained the same high level of 

 •excellence. 



Mr. Duddell has been carrying out experimental 

 research on the arc for the past five or six years, and 

 during the last two has, we understand, completed a 

 series of experiments on the vexed question of the resist- 

 ance of the arc. The questions dealt with in the paper 

 read last Thursday were mainly side issues which had 

 cropped up in the course of these researches. They 

 embody, however, a number of most interesting and 

 important results, many of which are suggestive of great 

 .possibilities. 



The paper was divided into two parts, the first dealing 

 with those cases in which the cause of the variation of 

 the current was in the circuit outside the arc, the second 

 with the cases where the cause was in the arc itself. 

 IJ.nder the first heading, Mr. Duddell gave the results 



NO. 1625, VOL. 63] 



of experiments which he had made on the rapidity with 

 which the P.D. between the electrodes of the arc, and 

 the light emitted by the arc itself and the crater on the 

 positive carbon, can follow variations of the current. 

 The results show that the rapidity is surprisingly great. 

 It is well known that with ordinary slow variations of the 

 current through an arc a rise in current is accompanied 

 by a fall in P.D. If the conditions of the arc were to 

 remain unchanged, the P.D. would rise with a rise of 

 current ; but Mr. Duddell has found that the conditions 

 of the arc can change as rapidly as 5000 times a second 

 or more, and that when the current through an arc between 

 solid carbons is suddenly increased it is only for the first 

 i/5oooth of a second that the P.D. rises with the current. 

 Messrs. Frith and Rodgers endeavoured, in 1896, to find 

 the resistance of the arc by superimposing on a direct- 

 current arc an alternating current having a frequency of 

 250 alternations per second, and measuring the change in 

 P.D. thereby produced on the assumption that at this fre- 

 quency the conditions of the arc did not change. The 

 results of Mr. Duddell's work show that a frequency of 

 at least 5000 alternations per second must be employed 

 before such an assumption is justified. 



It is remarkable also to find that the light emitted by 

 the arc is affected by such small and rapid variations as 

 Mr. Duddell found was the case. The light emitted by 

 the crater and the vapour column varies sufficiently dis- 

 tinctly for a photographic record to be obtained even 

 when the frequency of the superimposed variations in 

 current is as high as 4300 alternations per second,»and the 

 amplitude of the variation as low as 3 per cent, of the 

 mean. 



When the current through the arc is altered, a change 

 in the cross section of the vapour column is caused ; and 

 these changes, when the variations are rapid and periodic, 

 give rise to audible sounds. Mr. Duddell has found that 

 a variation of the order of one part in 10,000 from the 

 mean current will alter the vapour column sufficiently 

 to produce sound-waves. In this way an arc may be 

 made to act as a telephone receiver by causing the 

 varying currents in a telephone circuit to pass through 

 the arc. An experiment was shown at the meeting in 

 which the arc in the meeting room was used as a receiver 

 for telephone currents from a transmitter spoken into in 

 the basement of the building. The sounds were dis- 

 tinctly audible throughout the room, though the words 

 could hardly be distinguished beyond a distance of some 

 10 or 12 feet. These results were obtained with a cored 

 carbon arc — some 20-30 mm. in length and with a current 

 of about 10 amperes. 



The second part of the paper, dealing with changes of 

 current produced by the arc, was full of interest and 

 importance, and was illustrated by some very striking 

 experiments. Mr. Duddell first described some experi- 

 ments on the humming arc, in which he had found, by 

 means of curves obtained with his oscillograph, that the 

 P.D., current, and light emitted by the arc varied with 

 the same frequency, this frequency being identical with the 

 pitch of the note^emitted. With the-hissing arc Mr. Duddell 

 finds a double variation— a large slow one, which is due, 

 he considers, to the rotation of the arc as a whole, on which 

 is superimposed a small rapid variation in the P.D. and 

 current corresponding with the variation of the light 

 emitted by the crater, this variation being produced, as 

 Mrs. Ayrton has shown, by air obtaining access to the 

 surface of the crater. 



Perhaps the most remarkable points brought out by 

 Mr. Duddell in his paper were those relating to the 

 effects produced by shunting the arc with a condenser 

 and self-induction. He has shown that the arc, if it be 

 formed between solid carbons, when so shunted imme- 

 diately becomes intermittent and emits a musical note. 

 Mr. Duddell was led to this discovery by attempting to 

 use the arc as a generator of alternating current by 



