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Wetting current is a measurement used in electronics to describe how much energy is needed for the circuit to pierce a switch that has be oxidized. This oxidization usually is added on purpose via a film to help the circuit close, and it tends to be on the contacts. If the circuit is exposed to high humidity, then oxidization may form on the circuit; most engineers try to avoid this, because it can cause problems with the circuit. When the wetting current is not properly used, this can lead to a poorly functioning circuit.
Part of nearly every circuit is an oxidized film surrounding relays and switches. This oxidized layer is meant to add resistance to the current, making it more difficult for the circuit to get through the oxidization without the circuit being open. The wetting current is how much power the oxidization can take before it will forcibly be opened without the circuit being manually opened.
Since it helps stop the circuit, this oxidization generally is deliberately added around the contacts. There are different types of oxidization, and each is rated for a different amount of power. This film usually is chosen based on how much energy goes through the circuit on average, because using a thin film on a powerful circuit may not allow the circuit to close properly if the wetting current is set very high.
While oxidization generally is added on purpose, it also may inadvertently end up on the circuit. This normally occurs in environments with high humidity, and only if the circuit is directly exposed to the humidity. Failing to secure the circuit from this humidity can increase resistance on the circuit, making it unable to turn on, or the circuit may be weaker or run slower because less power is going through the relays.
As part of making a proper circuit, the wetting current should be carefully calibrated and used to make the circuit function correctly. Circuits have to open and close, or turn on and off, or the device will always be on. Without wetting current, the circuit would have no way of shutting off, because there would be nothing to stop the current from moving. This current usually is made according to the average amount of power going through the circuit, so stronger circuits typically will need stronger wetting currents to ensure the circuit can close.