34 Prof. H. F. Weber on the Inductions 



maximum. This limit will vary with each dynamo- machine and 

 each kind of lamp used. With the Wallace-Farmer machine the 

 limit appears to be reached when six lamps are connected up 

 in series. With the Gramme alternating machine and Jabloch- 

 koff candles the limit appears to be five lamps. Beyond 

 these limits the above laws will be true. It is this partial suc- 

 cess in multiplying the light that has led so many sanguine 

 experimenters to anticipate the ultimate possibility of its ex- 

 pensive subdivision — a possibility which this demonstration 

 shows to be hopeless, and which experiment has proved to be 

 fallacious*. 



IV. On the Inductions that occur in the Telephone. By Pro- 

 fessor H. F. WeberJ. {Communicated to the Zilricher 

 naturforschenden Gesellschaft, at the Meeting of July 1, 

 1878$.) 



MDUBOIS-RE YMOND has given, is his " Contribution 

 • to the Knowledge of the Telephone "§, the following 

 theory of the inductions that take place in the telephone: — 



The periodical variations of the electromagnetic potential P 

 of the magnetic masses in the telephone in relation to the path 

 of the current may, in the first approximation, be supposed 

 proportional to the outbendings of the iron membrane. If 

 the exciting membrane executes vibrations of the form 

 2A TO sin (27rnmt), then the periodical variations of the electro- 

 magnetic potential are given by the expression 



P — P = SB m sin (%Trnmt), 



where P denotes the value of P corresponding to the position 

 of equilibrium of the membrane. M. Dubois-Beymond neg- 

 lects the induction of the current-path upon itself as unessen- 

 tial, and sets forth as the really active electromotive force only 



* Vide Fontaine's ' Electric Lighting,' chapter xi. 



f Translated from a separate impression, communicated by the Author, 

 from the Vierteljahrsschrift der Zilricher naturforschenden Gesellschaft. 



\ Ten days later, on the lith July, 1878, M. Helmholtz transmitted to 

 the Berlin Akademie der Wissenschaften a memoir in which he handled 

 the same subject in the same manner. That already on the 1st July 1 had 

 made known the contents of the present memoir is evidenced, inter alia, 

 by the following, added by M. Hermann to his last paper in the Annalen 

 der Fhysik nnd Chemie, new series, vol. v. p. 91, on the 2nd of July: — 

 " Professor Fr. Weber, of Zurich, has succeeded in showing that the rela- 

 tion found by me is in harmony with the law of induction, and that the 

 latter has been wrongly applied in the theory which I have controverted. 

 He will shortly make a communication on this matter." 



§ Archivfur Physiologie, 1877, pp. 573, 582. 



