Page:Popular Science Monthly Volume 9.djvu/189

Rh turned, as it passes each rubber it is positively electrified. Facing the electrified glass is the row of points midway between the two rubbers. On these points the electrified glass acts by induction, attracting the negative and repelling the positive. In accordance with the principles already explained the negative electricity streams from the points against the excited glass, which passes on neutralized to the next rubber, where it is again excited. Thus the prime conductor is charged, not by the direct communication to it of positive electricity, but by depriving it of its negative.



If, when the prime conductor is charged, you bring the knuckle near it, the electricity passes from the conductor to the knuckle in the form of a spark.

Take this spark while the machine is being turned, and then try the effect of presenting the finger-ends, instead of the knuckle, to the conductor. The spark falls exceedingly in brilliancy. Substitute for the finger-ends a needle-point, you fail to get a spark at all. To obtain a good spark the electricity upon the prime conductor must reach a sufficient density (or tension, as it is sometimes called). To secure this, no points from which the electricity can stream must exist on the conductor, or be presented to it. All parts of the conductor are therefore carefully rounded off, sharp points and edges being avoided.

It is usual to attach to the conductor an electroscope, consisting of an upright metal stem, A C, Fig. 20, to which a straw with a pith-ball, B, at its free end, is attached. The straw turns loosely upon a pivot at C. The electricity passing from the conductor is diffused over the whole electroscope, and the straw and stem, being both positively electrified, repel each other. The straw, being the movable body, flies away. The amount of the divergence is measured upon a graduated arc.