It was known to the ancients that a fish called a torpedo existed in the Mediterranean which was capable of administering a shock to persons and benumbing them.  The torpedo, or “electric ray,” is found in the Atlantic as well as the Mediterranean, and is allied to the skate.  It has an electric organ composed of 800 or 1000 polygonal cells in its head, and the discharge, which appears to be a vibratory current, passes from the back or positive pole to the belly or negative pole through the water.  The gymotus, or Surinam eel, which attains a length of five or six feet, has an electric organ from head to tail, and can give a shock sufficient to kill a man.  Humboldt has left a vivid picture of the frantic struggles of wild horses driven by the Indians of Venezuela into the ponds of the savannahs infested by these eels, in order to make them discharge their thunderbolts and be readily caught.

Other fishes—­the silurus, malapterurus, and so on—­are likewise endowed with electric batteries for stunning and capturing their prey.  The action of the organs is still a mystery, as, indeed, is the whole subject of animal electricity.  Nobili and Matteucci discovered that feeble currents are generated by the excitation of the nerves and the contraction of the muscles in the human subject.

Electricity promises to become a valuable remedy, and currents—­ continuous, intermittent, or alternating—­are applied to the body in nervous and muscular affections with good effect; but this should only be done under medical advice, and with proper apparatus.

In many cases of severe electric shock or lightning stroke, death is merely apparent, and the person may be brought back to life by the method of artificial respiration and rhythmic traction of the tongue, as applied to the victims of drowning or dead faint.

A good lightning conductor should not have a higher electrical resistance than 10 ohms from the point to the ground, including the “earth” contact.  Exceptionally good conductors have only about 5 ohms.  A high resistance in the rod is due either to a flaw in the conductor or a bad earth connection, and in such a case the rod may be a source of danger instead of security, since the discharge is apt to find its way through some part of the building to the ground, rather than entirely by the rod.  It is, therefore, important to test lightning conductors from time to time, and the magneto-electric tester of Siemens, which we illustrate in figures 98 and 99, is very serviceable for the purpose, and requires no battery.  The apparatus consists of a magneto-electric machine at, which generates the testing current by turning a handle, and a Wheatstone bridge.  The latter comprises a ring of German silver wire, forming two branches.  A contact lever P moves over the ring, and is used as a battery key.  A small galvanometer G shows the indications of the testing current.  A brass sliding piece S puts the galvanometer needle