Electro-medical instruments and their management, and illustrated price list of electro-medical apparatus / by K. Schall.

  • Schall, K.
Date:
1900
Catalogue details

Licence: In copyright

Credit: Electro-medical instruments and their management, and illustrated price list of electro-medical apparatus / by K. Schall. Source: Wellcome Collection.

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    therefore electricity cannot spread equally over glass or wax, but remains localised. Substances which do not conduct electricity, such as glass, silk, ivory, oil, pure water, air, paraffin-wax, etc., are called non- conductors or insulators. Other substances, which allow electricity to pass freely, such as all metals, carbon, some minerals, etc., are called conductors. Between conductors and non-conductors there is, however, a third class of materials, which do not conduct electricity nearly as well, as, for instance, metals do, but which still conduct it decidedly to a certain extent; such as acids, salt and alkali solutions, etc. Such fluids are called half-conductors. Now to this class belongs the human body. It is of the greatest importance and convenience that there are bodies which conduct, and others which do not conduct electricity, for we are thus enabled to direct the electricity exactly to the spot where we desire its action, and to send it along metal wires which are supported by insulators, over any distance we like. By rubbing a glass stick, we have separated the positive and the negative electricity contained in the stick: the positive electricity remained in the glass bar; the negative passed through the rubbing cloth and the body of the rubbing person into the earth. In rubbing a metal stick we separate the two kinds of electricity in the same way ; but we can prove this only by insulating one end of the metal stick, for instance, by cementing it into a glass tube, and holding the glass tube in our hand while rubbing the metal. If we were to touch the non-insulated metal stick, the separated negative and positive electricity would get re- united again immediately through the body of the rubbing person, a nd would leave no trace at all. For a long time friction was the only means known to electrify bodies ; but about a hundred years ago, Volta and Galvani discovered another and far more convenient method to produce electricity, z.e., the simple contact of two different metals, and chemical action. They there- by gave the first impulse to the wonderful development of electricity which we have witnessed in our day. The electricity produced in this way has been called galvanic electricity, in honour of one of its discoverers. GALVANIC ELECTRICITY. Electro-motive Force.—If we immerse a piece of metal in some fluid which has the power of acting chemically on the metal, the two kinds of electricity are separated too. The power which disturbs the normal condition, and separates the positive from the negative electricity, is called electro-motive force. The E.M.F. is in some proportion to the intensity of the chemical action, but is independent of the size or shape of the metal. If we immerse zinc in diluted sulphuric acid, the zinc becomes negatively, the sulphuric acid a little positively electric. Copper im-
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