Marie's choice of research subject, and her subsequent discovery of two new radioactive elements, owed much to the groundbreaking work of her husband, Pierre, and his brother, Jacques. The brothers proved that a crystal under mechanical stress accumulates an exact, reproducible electric charge that increases as the crystal is further deformed--a phenomenon known as the "piezoelectric effect." Pierre and Jacques used this discovery to construct a new type of instrument that could precisely measure extremely small weights and energy outputs. Their "quartz piezo electrometers" consisted of a tray suspended from a quartz crystal and hooked up to a magnetically balanced needle. Putting weight on the tray stressed the crystal, and the resultant electric charge moved the needle.[1]