ELECTROSCOPE, an instrument for de tecting or determining the presence of elec tricity upon a conductor, and showing whether it is positive or negative. It has many forms, of which the simplest consists of pith balls hanging from silk threads in a dry, closed glass case. On contacting with an electrified body the piths are excited and swing apart. In the gold leaf electrometer there is a wide-mouthed vial stoppered with a cork, through the centre of which a metal rod passes into the middle of the vial. There is a brass knob at the top of this rod and its lower end is bent or hooked to support a narrow strip of gold • foil, which is folded in equal lengths over the hook. When a rubbed glass rod is brought near the brass knob, negative electricity is attracted and posi tive electricity is repelled to the gold leaves, which diverge by reason of the repulsion of the similar electricity on the leaves. To show the kind of electricity with which the leaves are charged, or with which another body may be charged, a finger is placed on the brass knob while yet the glass rod is near it. This allows the positive electricity to escape. When next the finger and then the rod are removed the negative electricity is dispersed over the gold leaf system and the leaves again diverge. If
now, while the leaves are charged with negative electricity, a negatively charged rod be brought near the lamb, the leaves tend to diverge still farther. If a positively charged rod is used the negative electricity in the leaves is at tracted and the leaves tend to collapse.
Electroscopes of this general type are now utilized, on account of their great susceptibility to the presence of electricity, to detect and measure the radioactivity of weakly radiating substances like uranium and thorium, advantage being taken of the ionizing properties of such substances by which the particles of gases be come carriers of electric charges proportional to the radioactivity of the substances. For this purpose the gold leaf system is placed in metallic connection with the upper plate of a small air condenser, on the lower plate of which is spread a layer of the radioactive material. A source of electromotive force is connected with the lower plate and the movement of the gold leaf is noted. As the rate and extent of this movement vary with the radioactivity of the substance, comparisons can be readily made of different substances or with a standard. See