Page:Popular Science Monthly Volume 4.djvu/474

458 Collecting Apparatus.—Dellman's apparatus for collecting atmospheric electricity is a hollow brass or copper ball about six inches in diameter, with a stem of metal. The metallic stem rests in a metallic tube, but is isolated from the tube by shellac. This apparatus is attached to a pole almost thirty feet long. This pole is drawn by a windlass up the walls of a house to the top of the roof. The operator then touches the stem of the ball with a piece of brass in the shape of a half-moon. This charges the ball with electricity. The pole is now let down at once, and the collecting apparatus is brought in control with the measuring apparatus. The electricity which is thus collected in the ball is developed in it by induction. The natural electricity of the ball is separated by the surrounding atmospheric electricity into positive and negative electricity. One of these goes to the lower part of the ball, the other remains in the upper part. Atmospheric electricity is usually positive. The natural electricity of the ball being decomposed, the negative is attracted to the upper and the positive to the lower part. When the operator touches the stem of the ball with the piece of brass, the positive electricity is conducted through his body to the ground, and the negative remains in the ball.

When, therefore, the electrometer shows negative electricity, it indicates positive electricity in the atmosphere, and vice versa.

It has been shown that there are two daily tides of positive atmospheric electricity—the high tides between 9 and 12 and between 6 and 9 ; the low tides between 2 and 5  and 1 and 5  The annual variations are fully as marked as the diurnal; the quantity of positive atmospheric electricity being greatest in the winter, least in the summer. Dr. Wislizenus found that, in 2,124 observations made at regular hours, the atmospheric electricity was 2,046 times positive and but 78 times negative. Of the 78 times, 30 were connected with thunder or hail storms, or by thunder and lightning, 23 by common rains, and 20 by high winds and gales without rain, thunder, or lightning, 4 by snow, and 1 by fog.

According to Herschel, out of 10,500 observations at the Royal Observatory, only 364 showed negative electricity. The remainder, 10,176, all showed positive electricity. Negative electricity was usually attended with rain.

It seems, therefore, that the chief cause of a condition of negative atmospheric electricity is storm, and especially thunder-storms, and that at all other times positive atmospheric electricity prevails. In very many cases this change to negative electricity takes place shortly before the storm approaches; during its progress there may be—especially in thunder-storms—rapidly-repeated alternations of positive and negative conditions, followed by an equilibrium, or by positive electricity.

Dr. Wislizenus, of St. Louis, also found that snow-storms and fog