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We present an experiment that illustrates the well–documented but often misinterpreted idea of induced currents: that an electron drifting toward an electrode induces a current on that electrode as soon as the electron starts moving, not when it arrives at the electrode. To illustrate this phenomenon experimentally, we use a gas–filled parallel plate ionization chamber and a collimated 241Am alpha source, which produces a track of ionization electrons at a constant height (drift distance). With a variable–height platform under computer control, we can move the alpha source to vary the ionization electron drift distance, but keep the number of ionization electrons fixed. We find that the detected signal from the ionization chamber grows with the electron drift distance, as predicted by the model of charge induction, and in conflict with the claim that the output signal is produced by the arrival of the electrons at the anode plate. We also show that the interplay between the drift time of the electrons and the timescale of the readout electronics impacts the detected signal, and that this effect can be used to measure the drift speed of electrons in the gas.


This is the author's accepted manuscript, dated Oct. 17, 2013. The final version will be published in 2014 in The American Journal of Physics and will be available here: