This liquid sensor from Comus has two gold plated metal probes fitted close to the body. When the probes detect the presence of liquids the change in resistance will activate the internal reed switch contacts and cause the contact to close and the circuit to be made. * Liquid drains away from the probes and resistance will drop and the switch contacts will open * Liquids of very poor viscosity may not drain away from the probes and will cause the switch contacts to remain closed