Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.
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must have large and complete contact. Let this be repeated first without, and then with the interposition of the portion of body to be tested, and a very exact relative degree is obtained of the resistance of that given organism. Direction of the Current. In the original voltaic pile, the positive pole, we have seen, is at the top of the pile, which is zinc ; while the bottom of that pile is consequently positive, which is copper. This lata holds good with all dry, or moist, piles and batteries; as by the law of electro-motive force, positive electricity is driven from the copper to the zinc, and hence the zinc is forced to be positive and the copper negative. But, mark — this is not true of our ordinary liquid batteries ; for where the metals are plunged into separate vessels, as is the case in the constant batteries, the direction of the current is exactly the reverse. In these bat- teries, the positive current travels from the zinc through the liquid of the battery to the copper, and then from the copper to the zinc of the next pair, and then through that liquid to the copper there contained, and so on through all the series of pairs in the cxips or jars of the compound battery ; so that the copper is always the positive pole in these constant batteries, and the zinc of course is the negative pole. This is even so in a Berze- lius's or a Smee's battery, where there is a single pair in a single liquid. Therefore, in the Berzclius's and Daniell's bat- teries, the positive pole will be formed by the copper ; in Grove's or Smee's batteries, by the platinum or silver ; in Bunsen's battery by the carbon ; while the zincs form the negative pole in each and all of these batteries. To fully appreciate the kind, nature, and difference of bat- tery or chemical electricity, and to have a rational view of these currents, we must set out with this law, namely: where a liquid attacks a metal, the liquid becomes charged with positive elec- tricity, and the attacked metal becomes negative. The positive of the fluid is at once taken and conducted by the less oxidiza- ble metal that is plunged in the same, and thus conveyed along the conducting wire, back to the oxidizing negative plate in the