Measure the rate the lamp adds heat by measuring the current through it
and the voltage across it. The product of current and voltage gives Watts,
the rate energy is added in Joules per second. The slope of your temperature-time
graphs is proportional to heat flow. With the lamp off, the slope gives
the heat loss rate. With the lamp on, the slope gives the difference between
the known rate the lamp adds energy and the heat loss rate. From this, calculate
the heat loss rate.
Minimize heat loss by adding insulation to the incubator. Does the rate
at which the bulb turns on and off change. Determine how much the heat loss
and heat gain rates change. How much difference does the fan make?