The context is the measurement of engine speed on a vintage car that uses a classic Kettering ignition system. Essentially, a voltage pulse of tenths of kV is generated on a high tension (HT) lead. The frequency of the pulses can be used to calculate engine speed.
I have seen two methods of measuring these pulses with instruments:
- An oscilloscope with a 1000:1 attenuation probe. The probe tip is a clamp that encloses a segment of the HT lead for measurement
- A RPM gauge with a wire wrapped ca. 10 turns around a segment of the HT lead for measurement.
The second type of measurement, in particular, would appear to be inductive, as we're measuring through a coil, but I'm told nevertheless that it's capacitive coupling.
Could someone elaborate on how capacitive coupling works in these two measurement setups?