CHAPTER XIV.
INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.
Fig. 90.
131. Atmospheric Electroscope.—The simplest instrument for ascertaining at any time the electric condition of the atmosphere is an electroscope composed of two equal pieces of gold leaf, suspended from a brass support, and insulated, as well as protected from the movement of the air, by a glass covering. Fig. 90 represents such an instrument. The cap of the brass support is fitted for the reception, in the vertical direction, of a metallic rod, not less than two or three feet in length. The top of the rod carries a clip. The instrument acts according to the law, that bodies similarly electrified repel each other; but when dissimilarly electrified, they attract each other. To make an observation, the instrument is placed in the open air, and a lighted piece of cigar fusee, or touch-paper, is fixed in the clip. The electricity of the air is collected by the substance undergoing combustion, and conducted by the rod to the gold leaf; and the pieces, being similarly electrified, separate more or less according to the amount of electricity present. The kind is determined by the effect of either an excited stick of sealing-wax or rod of glass upon the electrified gold leaf. A rod of glass, when rubbed briskly with a silk handkerchief or piece of woollen cloth, becomes positively electrified, or excited, as it is termed. A stick of sealing-wax, similarly treated, acquires the negative state. If, therefore, an excited glass rod be presented to the cap of the instrument, and it cause the pieces of gold leaf to diverge still further, the electric state of the air must be analogous to that of the glass, that is, positive; if they approach, it is negative. On the contrary, if a stick of sealing-wax be used, the pieces will be repelled more apart if they have acquired negative electricity from the air; and they will converge if they have a positive charge.
By means of this very simple instrument, meteorological observers can readily ascertain the electric condition of the lower air at any time.
Note.—A book containing strips of gold leaf is sent with the Electrometer to replace the gold leaves when torn or broken in use.
To mount fresh gold leaves, unscrew the brass plate to which is attached the rod supporting the leaves; then moisten with the breath the flat piece of brass, and press it gently down on one strip of gold, whilst the book is only partly opened; the second leaf is attached in the same manner.
132. Volta’s Electrometer is similar to the instrument just described, except that instead of gold leaf two light pieces of straw, or two pith balls, are freely suspended from the conductor; the amount of the electric charge being estimated from the degrees of divergence, shown by a graduated arc.
133. Peltier’s Electrometer is a much superior instrument in point of sensibility. A tall glass tube an inch or more in diameter, is connected to a glass receiver, mounted on a base fitted with levelling screws. At the top of the tube is formed a globe from four to five inches in diameter, which is thickly gilt on the exterior, so as to form a good conducting surface. A wire passes from the ball down the tube into the receiver, where it is bent up, and ends in a steel point over the centre of the base. A bent wire, carrying a small magnetic needle, is balanced on the steel point, so that the magnet, with the fine wire, arranges itself horizontally in the direction of the magnetic meridian. If any cloud or portion of air in the neighbourhood be in an electrical state, it will act by induction upon the gilt ball, and the needle will be deflected from its north and south direction.