Cloud chamber with dry ice and alcohol shown on TV camera.

Cloud chamber with dry ice and methanol shown on TV monitor. This Wilson continuously sensitive cloud chamber can work for the entire class period, displaying tracks of various radioactive decay products. It takes 5-10 minutes to reach a sensitive stationary activated state. A felt ring located under the top cover is soaked with methanol which will give off a constant vapor when placed on top of the glass cylinder of the chamber. The cover plate is then clipped on with spring clips. A reservoir under the base is packed with dry ice (about 180 mm in diameter and 40 mm thick). Cooling the base creates a supersaturated atmosphere a few centimeters above the base of the chamber that causes methanol to condense out on ionized trails left by _ and _ rays. Single traces of high-energy electrons caused by cosmic radiation are readily visible. There is a hole in the chamber side where weak radioactive samples can be inserted. Warning: don't insert strongly active substances like radium: it will disturb or temporarily ruin the proper functioning of the chamber. In a darkened room, the slide projector light source helps to illuminate the ion trails. A 'clearing field' potential difference of 2000 volts is placed across the top and bottom of the cylinder to clear the air of ions so that new tracks can be seen. A TV camera can be used to display the ion trails on a monitor.