Brownian Motion of Smoke Particles

Smoke cell under microscope; smoke particles seen bombarded by air molecules.

What it Shows

Brownian motion shows direct evidence of the incessant motion of matter due to thermal energy. Here we use the random bombardment of smoke particles by air molecules.

How it Works

The CENCO Brownian Movement Apparatus consists of a metal chamber with a glass viewing window on top and a lens on one side (see figure 1). Smoke from a piece of smoldering rope or match is drawn into the chamber through an inlet tube by squeezing the rubber bulb. The chamber can be illuminated by laser light shining in through the lens, which concentrates the light at the center of the chamber. The smoke cell sits on a microscope stage, and focusing down through the top viewing window reveals the smoke particles as tiny bright stars against a dark background, jiggling about as they are bombarded by air molecules. Superimposed on this rapid random motion is directional drift due to convection currents.

schematic diagram of apparatus, top view and also in action
figure 1. CENCO Smoke Cell features

Setting it Up

Illuminate the chamber with a HeNe laser and beam expander to make use of the whole lens. Using a smoldering cotton rope, draw smoke into the chamber and focus midway down the chamber with 50x power magnification (this brings the objective very close to the viewing window, so use the fine focus). The smoke particles will appear as tiny bright red specks or stars. As the depth of field is narrow, the particles will appear and disappear above and below the focusing plane. To view the demo, use the CCD camera mount atop the microscope and a monitor. Smoke activity in the cell will die down after a few minutes, so keep the smoldering rope handy.


A strong directional lamp can be used instead of laser illumination, but the laser works brilliantly (so to speak). We also found a small cotton rope smolders much better than a match. The smoke cell is CENCO catalog #60921-02. A point of historical interest, when Robert Brown noticed pollen grains jiggling about in water, he concluded that they were alive, and did not attribute the movement to molecular bombardment.