A practical introduction to medical electricity : with a compendium of electrical treatment translated from the French of Dr. Onimus / by A. de Watteville.

  • Armand de Watteville
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: A practical introduction to medical electricity : with a compendium of electrical treatment translated from the French of Dr. Onimus / by A. de Watteville. Source: Wellcome Collection.

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    same circumstances, diflferent bodies give different results, and we are led to the conclusion that every substance does not conduct an electrical current equally well; in other words, that besides the diameter of a conductor, its specific resistance must be taken into account. In order to make this clearer, we may suppose our cisterns to be connected through a number of pipes of the same diameter, but packed with bits of sponge, or pumice, some very loosely, others again very tightly. It is evident that the amount of water transmitted by each will be inversely proportional to the degree of tightness of the packing, that is, to the resistance offered in each to the flow. Exactly the same obtains for bodies considered as electrical con- ductors : some transmit the current freely, some with greater diflfi- culty, some very sparingly indeed. So besides the diameter of a wire, for instance, the metal it is made of, must be taken into account, and its specific resistance allowed for. Metals are the best conductors. Even the worse conducting ones among them, such as mercury or bismuth conduct far better than non metallic bodies. Carbon conducts fairly well. Acids and solutions of salts have much less resistance than pure water. Bodies such as resins, india-rubber, silk, glass, dry air conduct so badly as to deserve the name of Insulators. The human body is a bad conductor—the dry epidermis almost acts as an insulator. Let us now put the two ideas of electro-motive force and resis- tance together in the shape of a formula. The current of electri- city will be directly proportional to the electro-motive force, and indirectly proportional to the resistance in the circuit. Or mathe- matically, C = |. This is Ohm's law, the foundation-stone of elec- trical science. Let us consider R a little more closely. It represents the resist- ance in the whole circuit. In travelling from the positive to the nega- tive pole of a cell, the current encounters the resistance of the wire, human body, etc., which are included in that part of the circuit. This we call the external resistance. But inside the cell what takes place ? The current, in order to complete the circuit, must flow from the positive to the negative metal, and there also it encounters a resistance, viz., that of the liquids and diaphragms interposed. This we call the internal resistance, and our equation C = ^ becomes C = rt^-rt- where R' is the resistance in the battery, and R that outside. We shall see that R' is often a most important element in electrical matters, and one which is often wholly lost sight of. Before proceeding any further we must pause to consider what actual quantities the letters C, E, R represent in the equation just a great defect in the old-fashioned text-books of electri- city, and with them in electro-therapeutical treatises, that they con- vey no definite idea by the enunciation of Ohm's law on account of
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