‎"HELMHOLTZ, HERMANN von. - MEASURING THE SPEED OF ELECTRICITY.‎
‎Ueber die Dauer und den Verlauf der durch Stromesschwankungen inducirten elektrischen Ströme. (On the Duration and Course of the Electrical Currents induced by Variation of an Inducing Current).‎

‎Leipzig, Johann Ambrosius Barth, 1851. Without wrappers as issued in ""Annalen der Physik und Chemie. Hrsg.von Poggendorff"", Bd. 83, Viertes Stück.(= Heft No. 8 of 1851). (The entire issue offered (Heft 2 of vol. 83 with titlepage to vol. 83). Pp.469-600 a. 2 engraved plates. Helmholtz's paper: pp. 505-540. Clean and fine.‎

Reference : 43324

‎First appearance of this paper in which Helmholtz for the first time measures and gives the mathematical formula for the duration of induced electrical currents and thus distinguished with mathematical accuracy from those due to nerve-action.""In (the offered paper) he begins by stating a mathematical law, which he had verified by a long and difficult series of experiments. By means of this law F. neumann was enabled to solve the problem he had previously laid aside, as to the distribution of current in a copper disk rotating below the two poles of a magnet......After Helmholtz had extended this exponiential law mathematically to divided circuits, he tested it experimentally by means of a new type of galvanic contact-key, which madeit possible to vary the interval between the opening and closing of any current, as required.""(Koenigsberger ""Hermann von helmholtz"", pp. 79-80).‎