Directions for classwork in experimental physiology / by Sir Edward Sharpey Schafer.

  • Sharpey-Schäfer, E. A. (Edward Albert), Sir, 1850-1935.
Date:
[between 1915 and 1935?]
Catalogue details

Licence: In copyright

Credit: Directions for classwork in experimental physiology / by Sir Edward Sharpey Schafer. Source: Wellcome Collection.

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    “amalgamated ” by rubbing its surface with mercury after it has been cleaned by dipping into dilute sulphuric acid. Other constant cells are Grove’s, where the positive plate is platinum and is plunged into strong nitric acid, separated from the sulphuric acid containing the zinc plate by a porous partition ; Bunsen’s, which is similar to Grove’s, but with a positive plate of carbon; Leclanche’s (Fig. 4), in which the acid is replaced by chloride of ammonium and the place of the positive plate is taken by carbon, which is surrounded by manganese dioxide • and Grenet’s, where carbon again forms the positive plate, but with a single fluid (bichromate of potassium dis- solved in dilute sulphuric acid), in which both plates are immersed. The so-called “ dry ” cells are modified Leclanches. The negative plate in every one of these cells is zinc.1 Electrodes.—The wires used in physiological experi- ments must always be insulated with gutta-percha or rubber, or with silk or cotton; in the latter case the in- sulation is rendered more effectual by dipping the covered wire into molten paraffin. For experimental purposes it is usual to place the ends of the wires (which must be clean and free from the insulating material) in some sort of holder, so that they can be more readily applied to the tissue which is to be investigated; these ends are usually termed the electrodes.'2 They are often made of platinum set in a vulcanite holder; but a pair of pins with fine wires soldered to their heads, constitutes a readily improvised and efficient pair of electrodes for most class purposes. Such pin-electrodes can be passed through a small cork, or joined in parallel fashion with Chatterton’s cement, with their cleaned points projecting for a few millimetres. Such an arrangement enables them to be conveniently handled. To determine which of the two electrodes in any case is the anode and which the kathode, they may be placed in contact with a piece of blotting-paper moistened with starch solution containing iodide of potassium (pole-testing paper). Iodine is set free at the anode and turns the starch blue. Feeble differences of electrical potential are Fig. 5.—Pin- electrode. Fig. 6.—Simple cork electrode- holder. 1 The accumulator or storage cell, with plates of lead and lead covered by lead peroxide, is also often used in physiological work. 2 The term electrode means literally the “ path ” of the electric current, and in this sense the wires throughout are electrodes. But it has come to mean technically the ends of the wires which are used to apply the electric current to a given object (such as an animal tissue).