=Electroscopes.=--These are instruments for detecting whether a body be electrified or not, and indicating also whether the electrification be positive or negative. The earliest electroscope devised consisted of a stiff straw balanced lightly upon a sharp point; a thin strip of brass or wood, or even a goose quill, balanced upon a sewing needle will serve equally well. Another form of electroscope is the pith ball pendulum, shown in figs. 2 and 3. When an electrified body is held near the electroscope it is attracted or repelled thus indicating the presence and nature of the charge. =Gold Leaf Electroscope.=--This form of electroscope, which is very sensitive, was invented by Bennet. Its operation depends on the fact that _like charges repel each other_. [Illustration: FIG. 11.--Gold leaf electroscope; it consists of two strips of gold foil suspended from a brass rod within a glass jar. Used to detect the presence and sign of an electric charge.] The gold leaf electroscope as shown in fig. 11, is conveniently made by suspending the two narrow strips of gold leaf within a wide mouthed glass jar, which both serves to protect them from draughts of air and to support them from contact with the ground. A piece of varnished glass tube is pushed through the cork, which should be varnished with shellac or with paraffin wax. Through this passes a stiff brass wire, the lower end of which is bent at a right angle to receive the two strips of gold leaf, while the upper end is attached to a flat plate of metal, or may be furnished with a brass knob. When kept dry and free from dust it will indicate excessively small quantities of electricity. A rubbed glass rod, even while two or three feet from the instrument, will cause the leaves to repel one another. If the knob be brushed with only a small camel’s hair brush, the slight friction produces a perceptible effect. With this instrument all kinds of friction can be shown to produce electrification. The gold leaf electroscope can be further used to indicate the _kind_ of electricity on an excited body. Thus, if a piece of brown paper be rubbed with a piece of india rubber, the nature of the charge is determined as follows: [Illustration: FIG. 12.--Distribution of electrification on a charged hollow sphere. If an insulated conductor _d_ be inserted through the opening in the sphere and brought in contact with the interior surface and afterwards carefully removed, it will be found, by testing with the gold leaf electroscope, that it has received no charge. If touched to the outside, however, the conductor will receive part of the charge.]