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To measure a propagation delay, we need a signal which, when it crosses a threshold in either direction, triggers the start and stop of the stopwatch.
In the quasi-stationary regime – that is, in any region where the wavelengths associated with the signals are considerably longer than the dimensions of the system – the variation in the signal is far too slow; at two different points, the signal is virtually the same, making it impossible to take a measurement by detecting a threshold.
As for doing so without a threshold, simply by observing a phase delay, the result is incorrect because, once the regime is established, it is a steady state resulting from an incident signal superimposed on the signals it induces or that are reflected, so that one can no longer speak of the departure and arrival of a single signal.
Whilst it is easy to measure the delay of a signal of several MHz in a 10-metre cable, as each half-cycle produces a pulse, it is virtually impossible to do so with a 1 kHz signal, whose slope will be virtually constant at every point unless the cable is several kilometres to tens of kilometres long.
The commonly held belief that, because the signal is square-wave even at 1 kHz, there is a sharp edge allowing a delay to be distinguished, is equally false. Even if we did not have standing waves – which requires the utmost experimental care, particularly for a spread-spectrum signal such as a square wave – the measurement taken will be of the propagation time of the high-frequency component – for example, the 2000–2 MHz harmonic – and we will know nothing about the speed of the 1 kHz component, which is certainly different. Not only do permeability and permittivity influence the propagation speed and are generally frequency-dependent, but the path taken will also differ; for example, a high-frequency component may pass through a coil via a capacitive effect, without following the wire.
Even less measurable would be the action/reaction time associated with a magnet falling into a tube, given that the magnetic field varies extremely slowly over time. The magnetic field created by the variation in the magnet’s flux within the tube is felt almost instantaneously by the magnet itself, which continues its path whilst immersed in the field resulting from the superposition of the two, his own and that of the reaction. In any case, a delay does not alter Lenz’s law. If there is propagation, it is because energy is carried along step by step, for example in a ferromagnetic circuit, with action and reaction occurring with the medium at every moment and at every point.
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Topic: What is Known about the TPU (Read 588909 times)
