[Illustration: 65. Electroscope. Scale about 1/7.]
Singer’s Atmospheric Electroscope is an efficient form of the instrument in which an ordinary gold-leaf electrometer has attached to its circular brass plate a brass rod two feet in length, with a clip at its upper extremity to receive a lighted paper or cigar fusee. The electricity of the air in immediate contact with the flame, causes, by induction, electricity of the opposite nature to accumulate at the upper extremity, where it is constantly carried off by the convection currents in the flame, leaving the conductor charged with the same kind and power of electricity as that contained in the air at the time of the experiment. The principle of this method was initiated by Volta, and has been extended and applied by Sir William Thomson in his Water-dropping Collector, which consists of an insulated cistern from which water escapes through a jet so fine that it breaks into drops immediately after leaving the nozzle of the tube. The result of this is that in half a minute from the starting of the stream the can is found to be electrified to the same extent as the air at the point of the tube. The scale value of each instrument has to be separately determined by repeated comparative experiments, and involves much delicacy of manipulation.
It is chiefly important for the ordinary observer to know that the occurrence of thunder and lightning should be always noted in the column headed “Remarks.”
[Illustration: 66. Lightning Conductor. Scale about 1/10.]
The destructive effects of lightning are too well known to need description here; the means, however, by which these may be averted demand a brief notice. Lightning when discharged from a cloud will always choose the better of any two conductors which may present themselves. The _stone_ of a church steeple and the _wood_ of a ship’s mast are bad conductors, but a galvanized iron wire rope is the best possible conductor, and accordingly this material is now generally employed for the purpose. A lightning conductor consists of three parts: 1, the rod, which extends beyond the summit of the building, 2, the conductor, which connects the rod with the underground portion, and 3, the part underground. The connection between each of these must be absolutely perfect, or the conductor will be faulty. The top is usually of solid copper tipped with platinum (Fig. 66), the body of galvanized iron rope, so as to adapt itself to the inequalities of the building and yet have no sharp turns in it, while the part underground is of solid iron rod. This latter portion should extend straight underground for two feet, and being bent at right angles away from the wall, should rest in a horizontal drain 10 to 15 feet long filled with charcoal, and be again bent downwards into a well of water. Should water not be available, it should rest in the centre of a hole 15 feet deep and 10 inches in diameter, tightly packed with charcoal, which, while conducting the electricity from the rod into the earth, serves also to preserve the iron from rusting.