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"They have been around Although Marconi first experimented with radio using a spark generator, it is no longer the fashionable method of transmitting information. Nowadays it is usually nothing more than simply annoyingly conveying to the listener that something has turned on or off. So in steps the first reason the smaller capacitors were used - spark quencing. These are usually little more than a few nF strapped across a set of contacts (usually in series with a small resistor to stop contact burn) and the spark is much reduced, and so is the interference on the wireless too! What was also noticed by these earlier engineers is that transients too were reduced. Seing as a transient was reduced by a cap across a set of contacts, it also made sense they could stop possible spikes making their way into a piece of equipment if used across the power carrying conductors and Earth. But in so doing they are not ridding the system of noise, just ensuring it bypasses the equipment and goes straight to the Earth. It must not be forgotten then noise has mainly a high-frequency component and, again, the higher the frequency the more the capacitor will conduct it from the Live and Neutral to the awaiting Earth. This is covered in more detail later in "Leakage in Technology". When Inductors and Capacitors Marry >>
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