Abstract
II.
DETERMINATION of Electro-motive Force.—Soon after the first experiments were announced, certain physiologists said that although the results of the action of light which I have just described may be observed, to say there was a change in the electro-motive force, as stated in the earlier communications, was not correct. That the effect was due to an alteration in the electro-motive force had been proved, but experimental details were reserved for the second part of the investigations. At first Sir William Thomson's electrometer was used, but the amount of electric potential to be measured was too small to get good results. Another plan of determining the electromotive force was adopted. This was the method introduced by Mr. Latimer Clarke, the eminent electrician, and described in his work on “Electrical Measurements.” The instrument devised for this purpose is called by him a Potentiometer, and measures electromotive forces by a comparison of resistances. Practically we found the Daniell's cell far too strong a battery to use as a standard of comparison. A thermo-electric junction of bismuth and copper was substituted for it. One end of the junction was constantly heated by a current of steam passing over it, the other being immersed in melting ice. The electro-motive force of this thermo-electric junction, as estimated many years ago by Regnault, is extremely constant, and is about the l/175th part of a DanielPs cell. By means of this arrangement the following results were obtained:—The electromotive force of the nerve-current dealt with in experiments on the eye and the brain of a frog varies from the l/300th to the l/400 of a Daniell's cell. Light produced an alteration in the electro-motive force. This change was, in many instances, not more than the l/10000th of a DanielPs cell. But though small it was quite distinct, and proved that light produced a variation in the amount of the electromotive force. By the same arrangement the gastroc-nemius muscle of a well-fed frog gave l/35th of a Daniel!; the same muscle from a lean frog which had been long kept, gave l/240th of a Daniel!; and the sciatic nerve of the well-fed frog l/480th of a Daniell. Dr. Charles Bland Radcliffe states, in his "Dynamics of Nerve and Muscle,"p. 16, that he obtained by means of Sir William Thomson's quadrant electrometer from a muscle a positive charge equal to about the tenth of a Daniell's cell, a much greater amount than ascertained by the method I have just described.
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D., J. The Physiological Action of Light 1 . Nature 15, 453–454 (1877). https://doi.org/10.1038/015453a0
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DOI: https://doi.org/10.1038/015453a0