Biologists investigated the rate of heat loss from endothermic animals under different environmental conditions using a physical model.
They used a hollow copper cylinder filled with heated water to represent the core of a mammal, wrapping it in a uniform layer of synthetic fur. The assembly was placed inside a wind tunnel where the wind speed could be varied. For each trial at a different wind speed, they measured the temperature drop of the water over a 10-minute period.
State one variable, other than wind speed and the material of the synthetic fur, that must be controlled in this investigation to ensure valid results.
