= Curie ionization-current measurement
{c}
<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.<Untersuchungen über die radioaktiven Substanzen by Marie Curie>[Chapter I(b), Messung der Strahlungsintensität, figures 1–3: apparatus, null compensation, saturation and field-dependent limitations]{ref}
\Image[/curie-ionization-chamber-and-charge-compensation.png]
{title=The functional measurement chain in Curie's ionization experiment}
{description=Original schematic of the plate, <electrometer> and compensation by <piezoelectricity> in the <Curie ionization-current measurement>. The charge traces use a stipulated constant $10\,\mathrm{pA}$ signal and an exactly opposing compensation ramp; they are not historical measured traces.}
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 $\Delta t=30\,\mathrm s$ gives
$$
\boxed{Q=I\Delta t=300\,\mathrm{pC}.}
$$
Supplying $-300\,\mathrm{pC}$ 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.
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