Where do I place the probe?
The diagram below demonstrates how the temperature gradient works by placing the probe in the centre point of the vivarium, by placing the probe in between the heat source and the cool end you will ensure that the heater can not drive the cool end past your set temperature. In this example the thermostat is set to 30.c, the heater will stay on until the probe registers 30.c - therefore the area between the probe and the heater will be 30.c and higher, and the area between the probe and the cool end will not be able to go over 30.c.
In the diagram below we show how not to place the probe under normal use, In this example the probe has been placed in what should be the cold end, if the temperature on the thermostat is set for 30.c then it will not switch off the heater until the cold end hits 30.c. Which means the hot end will be exceeding 30.c.. You can use this method of probe placement if you require a very hot basking spot at a temperature greater than is on the thermostat dial. But do so with caution as the vivarium temperature will never be any lower than whats set on the thermostat dial.
Getting the correct temperature gradients is a fine balance of ambient room temperature, probe placement and heater size. All three of these factors will need tweaking to achieve the correct temps.