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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]

A material exhibiting piezoelectricity, used in the Curie ionization-current measurement to supply a controlled electric charge.

Radioactive decay is the spontaneous transformation of an unstable atomic nucleus or nuclear state. Its timing can be unpredictable for an individual atomic nucleus while a large population has a reproducible average behavior. A constant nuclear decay constant $\lambda >0$ gives the familiar exponential decay law. The half-life describes how quickly the parent population falls, while radioactive activity describes its rate of transformations.^[ref]

This connection between randomness and a regular population law makes radioactive decay useful for measuring elapsed time and for understanding sources of radiogenic heating. It also distinguishes two questions in an experiment: what the source emits, and what an instrument records. Becquerel's uranium-radiation experiment used a photographic plate; the Curie ionization-current measurement measured an electrical consequence of radiation. Their readouts reveal different aspects of the same investigation.