to respond with "the probe goes on the heat source" really, that is not a good answer at all , there is alot of places ON the heat source where it could be put , it could be put on the back side of the heat pad like one person said , it could be put in the top side between the pad and glass like another person said, are u saying that those two spots would work the same i doubt it , what if i was using bulbs as my heat source , are u saying i should put the probe on the bulb cus that would be putting the probe ON the heat source ,