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.

    23/172 page 7
    FARADISM. Induction is the name given to a phenomenon exhibited by a wire or other conductor, in the neighbourhood of which a currenl or a magnet is placed. As long as no change occurs in the dis- tance and strength of the latter nothing occurs in the wire but as soon as any such change is set up, and as long as it lasts, electro- motive force is generated in the wire. In other words, whenever a current (or magnet) is (suddenly or gradually) rnade^ .^'^^^f near a wire (that is, whenever it is brought from an infinite dis- tance into its neighbourhood, or thence removed to an infimte dis- tance), and whenever any variation of strength occurs in the current (or magnet) while at rest, a certain amount of electro- motive'force is induced in the wire. This amount is absolutely definite in each case, depending upon the length of the wire influenced and the variation in the current. Suppose that in one case a current of I unit be made suddenly, that is in a very short time, say ^th of a second, in the wire AB; and in another case be gradually, say in one second, made, that is increased from o to i, we shall have in both instances a tota electro-motive force X generated in CD. But it is evident that at any instant of its existence the second current is lOO times weaker than the first, and though the same total quantity of electricity is con- veyed in the two cases, the effects in the first will be much greater owing to the concentration of that quantity in a very short time. Ohm's law governs induced currents, as we said, just as it does galvanic. As in dealing with a battery we have to consider its internal resistance, so in dealing with the induction coil we have to keep in mind the resistance of the wire itself. The resistance of wires vary directly with their length, indirectly with their diameter. The primary coil of faradising apparatus, for instance, being always made of a short thick wire has a much less resistance than the secondary coil which usually consists of a much longer and thinner wire. If we connect the wire CD with a galvano- meter, and changes of current occur in CD, we shall find that the needle is not always deflected in the same direc- tion. When the current from B to A is made, increased, or brought near, the current in- duced in CD is observed to flow in the opposite direction, viz., from C to D. The re- verse takes place when the current BA is broken, dimin- ished, or removed. If then by means of a key K, we alternately make and break the current, the needle Fig. I.
    digitised image 1
    digitised image 2
    digitised image 3