I have 240 volt baseboard heaters, I would like to up grade the antique thermostats. I checked the voltage at the thermostat and the wires were each 120 volts to ground. I checked voltage between the two wires and it was 240 volts. There are two termanals on the thermostat with one of these wires connected to each, no othe wire conections. How does this work, and what do I ask for at the hardware store. Thanks for the help!