The box goes in the cool end to tell you that temperature.
The probe goes over to the hot spot and reads that temperature.
*Hunter94 taping the thermometer probe to the bottom of the tank with the thermostat probe is a waste of a good thermometer because you do not need to know that temperature*
Sorry but its not a waste I do it just to make sure all the temps are right and the Thermostat isn't off or malfunctioning. Better safe then sorry.