Ampere's experimental proof of his law of induction: i₂ varies as i₁

Nine years before Faraday, Ampere (1885) observed the induction of current from one circuit to another, using an ingenious set-up. Unfortunately the published descriptions of his observations were badly worded and there has been some dispute as to what he actually observed. Other accounts of the experiment exist, however, and from them it appears that he though he had proved that i₂ varies as i₁. A modern reconstruction of his apparatus shows that it does in fact behave in this way. Instead of indicating pulses of secondary current at the moments of starting and stopping the primary current, it gives a constant deflection (implying a constant secondary current) all the time the primary current flows because it behaves like an overdamped ballistic galvanometer. In the circumstances his conclusion was justified, although he placed too much reliance on a single experiment.