114 



NA TV RE 



[November 30, 189^ 



giving excellent images when driven at a speed of one hundred 

 revolutions a second. The velocity was under the regulation to 

 within two per cent., and the millionth of a second represented 

 not merely an appreciable distance upon the negative, it was an 

 easily vicasiwable quantity. 



More than thirty years ago this German physicist stood, as 

 Wiiea'stone had done nearly half a century before him, in the 

 very gateway of the domain in which such activity has shown 

 itself of late — the domain of electrical resonance. He was not 

 only the discoverer along experimental lines, of the oscillatory 

 discharge and the viemonstrator of the existence of effects which 

 had already been embodied in the analytical work of Helmholtz, 

 Thomson, and Kirchhoff; he anticipated also many of the dis- 

 coveries of later investigators, and worked out quantitatively 

 the dependence of the rate of oscillation upon capacity, induction, 

 and resistance. Two of Feddersen's photographs have been 

 brought to general notice by reproduction in the fourth volume 

 of Wiedemann's " Electricilat." There is another set which I 

 consider even more significant, showing the increase in the 

 number of oscillations with diminishing resistance. It is copied 

 'u\.fac simile from the original plate in Fig. I. 



Another forerunner in the development of the methods which 

 it is my privilege to consider, was Prof. E. W, Blake, of Brown 

 University. His results, too, have become classical ; but I 

 refer to them because they are related in ways not always 



recognised in later work. We are all familiar with his interest- 

 ing photographs obtained by speaking into the mouthpiece of a 

 Bell telephone (Blake, Atn. Journal of Science, vol. xvi. (3) 

 p. 57), to the diaphragm of which was attached a rocking mirror. 

 Records obtained in a variety of other well-known ways, of 

 some of which I shall have occasion to speak, indicate that 

 these photographs do not give a complete trace of the vibrations 

 which go to make up the articulate utterances by means of which 

 they were excited, but the method is of interest in three par- 

 ticulars : — 



(i) It is one of the earliest attempts to substitute photo- 

 graphy for vision in the study of the transient phenomena of the 

 sound wave. 



(2) It substitutes a moving sensitive plate for the revolving 

 mirror.^ 



(3) It is a distinct forerunner of the method applied some years 

 later with somewhat better success by Froehlich to the analysis 

 of alternate current phenomena. 



Throughout the history of the study of the phenomena of 

 the time-infinitesimal we find the tendency to be to supplant 

 visual methods by methods of photographic record. One of 

 the most noteworthy achievements in experimental acoustics, 

 for example, is the application of the manometric ilame to the 



1 Stein, in a paper cited by Prof. Blake, Pogg. Ann. 159 (13/6), describes 

 a similar device, but it is difficult to ascertain from his paper to what extent 

 he succeeded with his experiments. 



study of sound waves. The drawings made by Koenig {Annalen 

 der Physik, vol. cxxii. p. 666 ; also in his "Experiences d'Acous- 

 tique, " pp. 47-84) in illustration and verification of the pheno- 

 mena of the organ-pipe and of the analysis of complex sounds, 

 have been admired by all of us ; and the repetition of his experi- 

 ments has delighted an entire generation of demonstrators ir 

 physics. In how many minds the question of the feasibility of 

 photographing the manometric flame has arisen I do not know, 

 but quite recently it has been shown by Doumer {Comptes 

 Rendtts, 103 and 105), and independently by Ernest Merritt 

 {Proc. A.A.A.S. 41, p. 82 ; also Physical Review, vol. i.) in a 

 paper read before this section, that l>y surrounding the sensitive 

 flame with a mantle of free oxygen (after the method of what 

 was once known as the " Budde " light), sufficient actinic in- 

 tensity may be obtained to ensure an excellent photographic 

 record on a rapidly moving plate. The results of such photo- 

 graphs applied to the analysis of vowel sounds give evidence of 

 the extraordinary fidelity of the sketches published by Kcenig. 

 They also afford a basis for the study of timbre of the sounds 

 to which they correspond, which is open to one objection only, 

 viz. to the uncertainty as to the influence of the inertia of the 

 diaphragm upon the character of the image. Of this source 

 of error I shall have more to say in connection with some other 

 researches. 



Other interesting examples of the study of the time-element 

 might be drawn from this field ; indeed, the science of sound 

 is of necessity largely made up of such work. The beautiful 

 photographs of vibrating strings by Menzel and Raps {Annalen 

 der Physik, N.F. 44, iSgr, p. 623), which are so fitting an 

 appendix to the earlier labours of Helmholtz {Die Tonemp- 

 findmigen, p. 137), may serve to illustrate the usefulness of the 

 method of photography on a moving plate. 



In the study of periodic phenomena two distinct methods of 

 investigation have been established. The first of these consists 

 in the isolation of a desired element of the cycle at each repe- 

 tition for as long a lime as may be necessary to obtain a satis- 

 factory record of the existing conditions. By the selection 

 successively of many neighbouring elements, we get in this way 

 at last the data from which to construct a complete diagram of 

 the cycle. 



This principle has been most fruitful in enabling us to analyse 

 periodic processes not easily approachable by more direct means. 

 The most notable application has been that which is commonly 

 spoken of as the " method of instantaneous contact," well known 

 to the student of alternating current phenomena. 



It is to Joubert ^ (1880) that we owe this ingenious adapta- 

 tion of the device of properly timed repetitions of instantaneous 

 observations of periodic phenomena (a principle which underlies 

 the phenakistoscope and similar well-known instruments). He 

 made use of it in the study of the changes of potential in the 

 circuit of the alternating current dynamo, and between the 

 terminals of the Jablochkoff candle. 



In the same year the method was discovered independently, 

 and applied to the study of the Brush arc-lighting dynamo by 

 B. F. Thomas.- Joubert pointed out the method of using the 

 quadrant electrometer in alternating circuits, also that the gal- 

 vanometer might be utilised. (" On peut mesurer cette inten- 

 site par I'electrometre mais on peut aussi employer le galvano- 

 metre puisque les contacts successive correspondent toujours a 

 une mcme phase du courant.") He discovered the retardation 

 of phase in the current curves of the alternating dynamo, and 

 the peculiar distortion of the curves in the circuit containing an 

 arc lamp, a matter more fully investigated at a later day by 

 Tobey and Walbiidge.^ Thomas during this first period in 

 the history of the Joubert method used a ballistic galvanometer 

 and condenser. 



The periodic phenomena of the alternating current circuit 

 have been among the most important to which the study of the 

 time-element has been applied, and it is to the method of in- 

 stantaneous contacts that we owe much of the progress of the 

 last thirteen years. It is interesting to note the extension of 



1 " Sur les Courants alternatifs et la force electromotive de Tare elec- 

 trique. ' Comptes Kendns. vol. xci. p. i6i, July 19, 1880. 



- " Observations on the electromotive forces of the Brush dynamo-electric 

 machine." (title only.) Proceedings A.A.A.S. vol. .xxix. p. 277 (1880). 

 Prof. '1 homas gave the results obtained, and described the method eleven 

 years later in a communication to the Institute of Electrical Engmeers, 

 entitled " Notes on Wiping Contact Methods for Current and Potential 

 Measurements," 1 ranslations of the American Institute of Electrical 

 Engineers, vol. ix. p. 263. 



•i " Investigations of the Stanley Alternate Current Arc Dynamo." Trans 

 Am. Inst. Electrical Engineers, vol. vii. p. 567. 



NO. [257, VOL. 49] 



