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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    between two insulated points, but also of keeping- up this difference when these points are connected through a conductor. As in hydrostatics we take the level of the sea as standard level for our measurements, so in electrics we take the potential of the earth as standard with which we may compare the potential of any point of body. In fact we assume the earth to be in a state of electrical rest, as we do the sea to be at a uniform level. And as the one is a practically unlimited reservoir of water, so the other is of electricity ; small additions or subtractions of fluid having no per- ceptible effect in altering- the level in either case. The potential of a body may therefore be defined as the excess or delect of its potential above or below that of the earth. In illustration of this conception we may adduce the thermometrical measurements of temperature. Our Zero point having- been fixed, as that of the freezing- point of water, we measure the temperature of bodies by refering- them to the standard so chosen. As then we have levels above and below that of the sea, and temperatures above and below freezing point of water, so we have potentials above and below the earth's potential; the former are called positive potentials, the latter neg-ative potentials. When, therefore, we say that a point is at a positive potential, we imply that if connected with the earth, electricity will flow from it to the earth, and, conversely, that if a point is at a neg-ative potential, electricity will, under the same circumstances, flow from the earth to the point. It will be observed that, so far, the expression of the potential, positive, or negative, of a body coincides with the statement that it is charged with positive or negative electricity. But it frequently happens that we have to compare the potentials of two points irrespective of their relation to the earth's potential; and we then speak of them as being positive and negative respectively, though both may be, absolutely speaking, positive, or both negative. But this does not constitute an exception to the general law that the flow of electricity always takes place from the higher to the lower potential; from the more positive to the less positive, if both are above, from the less negative to the more negative, if both are below the earth's potential. Exactly the same thing occurs in the case of temperatures; heat flows from the warmer to the colder point, whether both be above, or both below zero. Between two cisterns at different levels, either above or below that of the sea, the stream invariably follows the same direction. If, by means of an appropriate instrument, we test the terminals of a galvanic element, we find that the wire connected with the copper is electri- fied positively, and the wire connected with the zinc, negatively: hence, the names of positive and negative poles of the cell; and the direction of the current which flows in a conductor, uniting the two poles, is accordingly found to be from copper to zinc.^ When the two wires are placed in mutual contact, or in contact with a conductor, we sa> that the circuit is closed, implying by this expression that the current of electricity does not simply flow from positive to negative pole in the wires, etc. external to the cell, but that within the cell itself there is a current completing the
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