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domnic:
Following are photos of some potential relays (voltage, can also be current) that probably don't fit your definition of a potential relay.
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The left relay is a potential relay. This oscillates rapidly to adjust the average current to the field of a DC generator. It is the voltage regulator relay. This is about 60 years old and for a 6 V system. Probably a 52 Mercury.
The middle relay is a current limiting relay that also oscillates when necessary to adjust field excitation to limit maximum load current on the generator. When in current limiting it is obvious that voltage regulation is lost.
The right relay is a combination voltage and current relay, commonly called a cut-out relay, used to prevent the battery from backfeeding the generator when the generator voltage is too much below the battery voltage. Today a diode might be used, but the cut-out relay might waste less power.
My classmate's father invented an aspect of this regulator that reduced manufacturing cost.
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These are reed relays. One without the reed and the other with. Depending upon the winding the relay can be either a current or voltage relay.
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A very conventional P&B KUP relay, 24 V DC.
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A sensitive relay, low power. Probably not a good way to get sensitivity today.
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