The Curie electrometer and piezoelectric quartz

In 1880 the brothers Pierre and Jacques Curie discovered piezoelectricity: squeeze certain crystals, such as quartz, and they produce electric charge. They turned the effect into an instrument.

Marie Curie set it up in a cramped, damp storeroom at the School of Industrial Physics and Chemistry. The sample was spread on one plate of a condenser charged to 100 volts; the plates were 8 cm across and 3 cm apart. The rays made the air between the plates conduct, and a feeble current, around 10^-11 ampere, flowed to the electrometer. Hanging weights on the quartz plate released an exactly calculable counter-charge, and she adjusted the load, watch in hand, to hold the electrometer needle at zero. The weight and the time gave the current.

The method was more sensitive and more repeatable than Becquerel's photographic plates, and it turned radioactivity into a quantity. It made possible every step that followed: the atomic-property law, the thorium result, and the radiochemical hunt for polonium and radium. Later versions of the quartz electrometer were used at the Radium Institute to measure radium tubes for hospitals.