Genuine Aprilaire #50 24 volts, 0.5 amp current sensing relay. 

This relay will switch on a 24 vac load (0.5 amp max load) when 4 amps (blower motor) current are sensed.  Used to turn any humidifier on when indoor blower or air handler is energized. The pair of wires is 20 AWG with a 24" length. Dimensions 2-3/4" wide x 1-7/16" high x 1" thick.  The 24 vac rating does not refer to the blower voltage. The blower voltage can be 120v or 240v as only the current is sensed. The need to wrap the blower wire around the RP-50 is typical. 

 Installation instructions included.  Makes installing any humidifier easier than ever before.