Page:Electrical Engineering Volume 1.djvu/21

Rh plate or ball c. An electrified body, such as the rod d, brought into the vicinity of the electroscope, will cause the leaves to repel one another, due to the fact that they are both similarly electrified.

To determine the condition of a charge by the electroscope: First, charge the gold leaves with a known charge, such as that developed upon glass when rubbed with silk. The leaves will spread apart, being electrified with a positive charge. When they are thus charged, the approach of a body which is positively charged will cause them to open still more widely; while on the approach of one negatively charged, they will close together.

2211. The torsion balance is an instrument used to measure the force exerted between two electrified bodies. It consists of an arm or lever of some light insulating material, such as a straw or piece of wood, provided at one end with a gilt pith-ball n, Fig. 903, and suspended in a glass jar by a fine silver wire. The wire passes up through a glass tube and is fastened to a brass stopper b, called the torsion bead. The torsion head is graduated in degrees, and is capable of being revolved around upon the glass tube. Another gilt pith-ball m is fastened to the end of the vertical glass rod a, which is inserted through an opening in the top of the jar. A narrow strip of paper, also divided into degrees, encircles the glass jar at the level of the two pith-balls.