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Marie Curie's thesis describes powdered material on the lower plate of a capacitor, biased by a battery. Radiation made the intervening air conduct. The upper plate connected to an electrometer and a loaded piezoelectric crystal of quartz. Adjusting the load supplied a known compensating electric charge while keeping the electrometer at zero; charge divided by elapsed time determined the electric current. Voltage scans identified a collection plateau. Contact potentials affected weak-field readings; excessively strong fields could instead introduce additional ionization. Fixed geometry and plateau operation made comparisons meaningful.^[ref]

The null method turns a weak signal into a balance against a known quantity. In the diagram's chosen constant-current model, $I=10\mathrm{pA}$ for $\Delta t=30\mathrm{s}$ givesSupplying $-300\mathrm{pC}$ over the same interval leaves zero net electric charge at the sensing node. The electrometer detects imbalance; the compensating-charge calibration supplies the numerical measurement.

This electric current measures collected charge produced by ionization. Converting it to becquerels additionally requires the energy deposition and charge-collection response per transformation. The apparatus can compare preparations without already possessing that complete source-to-signal calibration.

A physicist and chemist whose investigation of radioactivity joined sensitive measurement to chemical separation. She moved from Warsaw to Paris to study, later becoming a professor of physics. The Curie ionization-current measurement shows the experimental craft behind the work: balancing a tiny electric current against a calibrated electric charge. She shared the 1903 Nobel Prize in Physics and received the 1911 prize in chemistry.^[ref]

One half went to Henri Becquerel for discovering spontaneous radioactivity. Pierre Curie and Marie Curie each received one quarter for investigating that radiation.^[ref] Becquerel's uranium-radiation experiment and Curie ionization-current measurement expose two complementary experimental routes into radioactivity.

A physicist who investigated magnetism, piezoelectricity and radioactivity. He built delicate balances, electrometers and piezoelectric crystals; this work on instruments helped make weak electrical effects measurable. With Marie Curie, he investigated radioactive substances under difficult laboratory conditions. The Curie ionization-current measurement illustrates how such instrumentation can turn a small signal into a quantitative comparison.^[ref]

By Marie Curie; translated with bibliographical supplements by Walter Kaufmann. Third unchanged German edition, Friedrich Vieweg und Sohn, Braunschweig, 1904; French thesis, 1903.

Cited passage: chapter I(b), measurement of radiation intensity, Figures 1–3. Gutenberg release 2011 and update 2020 are digitization dates.