(15 minutes or less)
Use one clip lead to attach the positive terminal of the battery to one end of the pencil lead, and the second clip lead to attach the negative terminal to the other end of the pencil lead. If you like, you may connect a simple switch, or a dimmer switch, in series. The switch makes using the device more convenient; the dimmer switch lets you vary the amount of current going through the carbon rod.
Fill the container with water and place the wires and pencil lead in it so that the pencil lead is positioned horizontally. Connect the two wires to the terminals of the battery, and allow the heating to start. Shine the projector through the liquid, projecting the light onto the screen or white posterboard.
(15 minutes or more)
Observe the convection currents. If you have a dimmer switch, vary the current and observe the effects of the various settings. If you are using a rheostat or variable resistor, you may have to try several settings to find which one works best. You can also vary the orientation of the pencil lead to see if this has any significant effect on the convection pattern. Add a few drops of food coloring and observe the effects.
Like air, water expands as it gets warmer and so becomes less dense. Since the water warmed by the current flowing through the carbon rod is less dense than the surrounding colder water, the warm water rises through the colder water to the surface, causing the food coloring to move along with it.
Since the cold and warm water have different densities, they have different indices of refraction. Light bends (refracts) as it passes from warmer to colder or colder to warmer. When light is bent onto an area of the screen, that area becomes brighter. When light is bent away from an area of the screen, that area becomes darker. The positions of warm and cold water are constantly changing, so the images projected on the screen shimmer and flow like heat waves in air.
A simpler method of doing this Snack, which allows people to perform it themselves, is simply to place a candle on a table and project this image onto a screen with a flashlight. The point source of a MiniMaglite™ projects clear images of convection when used on a small-scale desktop experiment. Changing the distance from a point light source to the candle will change the magnification of the image of the convection currents projected on the wall.