This depends on the controller, on the newer APS-C series controllers, EUR-5A controller and the PRO series controllers they are supplied with 24VDC across the red and black wires (black is ground). The control signal coming back to the melt control is measured between the white and black wires.
On the older APS-3A, APS-3B and the APS-4 controllers they are supplied with 24VAC on these controllers. The control signals are the same as for the newer melt controllers.
The supply voltage will vary slightly depending on the step down transformer and the voltage that is supplied to the control box.
When the sensor is warm and dry the voltage from white to black is very close to the supply voltage. When the sensors are wet and cold the voltage from white to black is less than 2 VDC