  In Rayleigh-B\'enard convection, a fluid volume is heated from below and
cooled at the top. Due to buoyancy forces, hot fluid rises and cold fluid
sinks, leading to an overall upward heat flux that has a power-law dependence
on the imposed temperature difference. Our experiment shows that, adding
sidewall heating modifies this power-law relation, thus brings control to this
classical fluidic system. Through careful flow, temperature, and heat flux
measurements, we investigate the effects of such side-heating to
Rayleigh-B\'enard convection and rationalize our observations with simple
relations that describe how side-heating controls the overall heat transfer.
Such control-response mechanism is conceptually analogous to that of electrical
transistors, and our "thermal transistor" may serve as a potential building
block for more complex thermal-fluidic circuits.
