To ascertain the quantity of electricity in a prime conductor or jar, an electrometer the most easily constructed and of the most general use has been invented by Mr. Henley—called the quadrant electrometer.—Of this we have given a representation in the frontispiece, (letter X.) It consists of a perpendicular stem formed at the top like a ball, and at the lower end with a screw, by which it is fastened to the prime conductor. A graduated semicircle of ivory, horn or stiff paper, is fixed near the uppermost end of the stem. A moveable index, made of a slender piece of hickory, extends from the centre of the graduated semicircle a little distance beyond its circumference, having a small ball of cork or pith at its lower extremity. When the conductor or jar is not electrified, the index is parallel to the stem, but when it is electrified the index recedes more or less, according to the degree of the electrization, which is marked on the graduated circle. A simple atmospheric electrometer was constructed by Mr. Cavallo in the following manner.— To the end of a common fishing rod, he affixed a slender glass tube covered with sealing wax, and having a cork at its end, from which two cork or pith balls were suspended by hempen strings. From the other end of the rod proceeded a flaxen or hempen twine a little longer than the whole rod and tube, with a pin attached to it, which was stuck into the cork at the extremity of the glass tube, for the purpose of taking off the insulation. The twine, to prevent its falling when the pin was pulled out of the cork, was attached to the rod, by a small string, running from it and meeting the rod at a little distance from the glass tube. To use this instrument, let the pin be pushed into the cork. Then, holding the rod by the extremity farthest from the cork balls, project it out, from a window in the upper part of the house, into the air, raising the end of the rod to which the balls are appended, so as to make an angle of 50° or 60°, with the horizon.—After having kept it in this situation a few seconds, by pulling the twine, detach the pin from the cork.—This leaves the electrometer insulated, and electrified with an electricity contrary to that of the atmosphere. Now draw the instrument into the room and you may examine the quality of the electricity, by applying the knob of a phial positively charged to one of the balls; if the ball is attracted by the knob it is negatively electrified—if repelled, positively electrified.