393. =Lightning-Rods.=--It was the discovery of Franklin which led to the custom of attaching lightning-rods to buildings. The object of a lightning-rod is to conduct any electricity in a cloud that may come over the building down into the ground. For this purpose the rod should terminate in the air in points, as these, as you saw in § 380, so readily receive the electric fluid. The rod should be separated from the house by wooden supports, and it should pass so far into the ground as to have its end in the midst of continual moisture. The points should be gilt, in order to preserve from corrosion, or they may be made of silver or platina. Lightning is very apt to go down in chimneys, as smoke is a very good conductor; and therefore it is well to have the rods go up by chimneys, especially if they are to have fire in them during the summer. Lightning-rods often undoubtedly are of service when there is no obvious passage of the lightning down them, by quietly and continuously receiving electricity upon their points, and passing it down into the earth. [Illustration: Fig. 277.] 394. =Galvanic or Voltaic Electricity.=--This form or mode of electricity I will barely notice here, reserving its full consideration for Part Second, where it appropriately belongs. The history of its discovery is interesting. The first dawning of Galvanism is to be found in an experiment noticed by Sulzer, a citizen of Berlin, in 1767. He states that if a piece of zinc be placed under the tongue, and a piece of silver upon it, on being brought in contact a metallic taste is perceived, and a shock is felt by the tongue. Sulzer attributed the effect to some vibratory motion occasioned by the contact of the metals, and, satisfied with this fanciful explanation, pursued the inquiry no farther. The statement excited but little notice until other facts of a similar character were brought out in 1790 by Galvani, professor of Anatomy at Bologna. He observed that the legs of some frogs, which had been obtained for his invalid wife, were convulsed, when near an excited electrical machine, on touching the nerves with a knife. In contrast with the example of Sulzer, he was led to examine the matter further. He found that the effect was produced when no electricity was communicated from the machine, by establishing a connection between the nerves and the muscles by some conductors. For example, when a strip of zinc was placed in contact with the nerve which goes to the lower extremities, and a strip of copper in contact with the legs, on bringing the two together at the other end the legs would be convulsed, being drawn up, as represented in Fig. 277 (p. 306). But Galvani did not get at the true explanation. He supposed this to be an exhibition of animal electricity, regarding the muscles as being a sort of Leyden jar, the nerve being the medium of communication with the inside. [Illustration: Fig. 278.]