Convection Cell (Thymol Blue Cell)

What it shows:

Here we set up a convection cycle, where we heat part of a fluid; it expands, rises, then cools and sinks. A two dimensional model for real convection cycles in the atmosphere, oceans or stellar interiors.

How it works:

The convection cell is made from two sheets of 0.5cm plexiglass (front clear, rear translucent), separated by a 0.5cm gap for the liquid, the indicator thymol blue. The sides are plexi and the top left open, but the bottom is sealed with a hollowed brass rod. The brass serves two purposes. Firstly as a heat source, seven 33Ω resistors take up one half the length of the rod (they should fit snugly). Connected to a variac and 110V DC, you can regulate the heating and convection rate.

To see the convection effect, thymol blue, which is ordinarily yellow, turns blue when ionized. A 100V potential difference is set up between the top and bottom of the cell; the top (negative) electrode being a wire across the cell, just below the liquid level. The bottom (positive) electrode is the brass rod. When the potential is applied, the thymol blue changes color along the wire at the top, and the blue will begin to circulate through the yellow bulk when the heat is on.

figure 1. Convection Cell

thymol blue

Setting it up:

Fill the cell with the aid of a funnel and stand it in a shallow dish (with all the silicon rubber sealant, it still tends to leak) You will need a lab clamp to hold it upright. Turn the variac on the heater up to 60V AC before the class starts to get a nice current circulating. When the lecturer is ready for the demo, s/he can turn on the 100V bias. Illuminate the cell from behind using a light box. View with a color camera.


There hasn't been another demo to attract as much criticism and complaint as this one. An essentially good idea has been very disappointing in action. By all means use it, but don't say we didn't warn you.