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. A graduated circle indicates the number of degrees of the deflection, which will be greater or less according to the tension of the electricity. To ascertain whether the electricity is positive or negative, a stick of shellac or glass must be employed, as already described.