It is important that teh heating surface of the hot plate be level. Otherwise, it will be necessary to add more oil to cover the bottom of the petri dish. A thin layer of oil is essential to the success of the experiment. Thin layers, on the order of 1 or 2 millimeter, do not exhibit significant convection currents as do layers that are much thicker. There simply is not enough room for convection currents to develop in thin layers.
Heat is conducted through the thin layer to the surface very quickly.
Since the lower and upper parts of the liquid are at nearly the same temperature, no convection currents develop. The demonstration is conducted with two petri dishes. Use the lids of both dishes for holding the oil and spice. To see the surface tension effects, sprinkle the cinnamon from a height of 20 or 30 centimeters to help it spread out evenly on the surface of the oil. Place the first dish on the hot plate and observe that patterns are produced by the cinnamon.