Fill this rectangular shaped glass tube with water, put a drop of colored dye in, and then heat one side of the glass tube. The resulting convection currents are conspicuous. The tube is 4-3/4" X 7-1/2".
Click here to see a video of this demo.
A metal box (23 cm x 10 cm x 10 cm) is closed at the front with a glass panel and has two glass lamp chimneys on top. A holder under one chimney lets you place a candle under it, warming up the chimney's base. The convection currents this causes can be seen by using a smoke source over the other chimney.