Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.
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The conductibility of bodies for electricity, which are so reck- oned, is not, however, an absolute property, for there is every grade of difference observed in different bodies; and these even vary in themselves at different times. Water is a conductor, but dry ice is a good insulator. Glass is reckoned a most ex- cellent insulator; but if it is damp, or is drawn out into a slim rod, or is heated to redness, it becomes a good conductor. Wax and tallow become conductors by melting them; but the metals, on the contrary, do not conduct so well when they are heated. The dry atmospheric air, being an insulator, prevents elec- trized bodies losing their electricity. Were the air a conductor, it would cause the electricity of every insulating body to be dis- sipated into the whole mass of the atmosphere, as does contact with the earth. This does take place when the air is moist, or where the apparatus is colder than the air. Therefore it is necessary to have a dry atmosphere, and a machine free from humidity, to make experiments or practice succeed well with friction electricity. It is necessary to be mindful, that, if any electric current of a given quantity and intensity is made to pass through several different wires, of the same sample of metal and make, and same diameter, but of different lengths, we shall see that the cur- rent loses power in proportion to the length of the wires tested. If, now, the current is made to pass through wires of exactly the same metal and length, but of different diameter, the power is greater in proportion to the diameter of the wires. For in- stance, a copper wire that is a hundred feet long and the tenth of an inch in diameter, will offer the same amount of resistance as another copper wire that is two hundred feet in length and only one fifth of an inch in diameter. The conductibility of bodies, therefore, does not depend upon their chemical nature only, but also upon their form. Nor are there any absolutely perfect conductors of electricity. Indeed, it may be said that all bodies through which a current may be directed offer a cer- tain greater or less resistance; and hence diminish its intensity according to their chemical nature and form. If a galvanometer be placed between the shortest poles of a