Diagrams of an electroscope (left) & an electrom- eter (right) - instruments used in early studies of radioactivity & subatomic particles. The elect- oscope is a box with a window. A metal rod passes through an insulating collar & has two thin sheets of gold foil attached to its end. When the rod is electrically charged, the gold leaves repel each other. If the air in the box is ionised, for instance by radiation, the charge on the leaves is neautralised & the leaves collapse together. The electrometer allows the amount of electricity to be accurately measured. Quartz fibres held under tension repel each other when charged; the degree of separation is measured with the microscope.