Jochen's High Voltage Page

CRT as a vacuum pump

For this vacuum experiment you don´t even need a pump: a cathode ray tube (CRT) from a TV, monitor etc. serves as a "vacuum reservoir". All else you need is a suitable discharge tube, some sealing stuff and a sharp, pointed drill or gimlet. The discharge tube is evacuated by opening a small hole in the CRT, through which most of the air inside the tube is sucked into the very well evacuated CRT.

The discharge tube should be neither too large nor too small, maybe 10-20cm long and 1-3cm in diameter. The larger the vessel, the worse the vacuum. The tube must be sealed air-tight at one end; the other end remains open, with a smooth and flat rim. One or two electrodes should of course be included, preferably at distant ends of the tube. The tube must be strong enough to withstand atmospheric pressure - I recommend clear plastic that doesn´t shatter in case it turns out to be too weak.

The hole through which the tube will be evacuated is pierced through the small metal plate where normally the HV supply is plugged in. For our purposes the plug must be removed, of course. The glass surrounding the plate must be thoroughly cleaned, as well as the rim of the open tube end. Some kind of flat rubber o-ring is attached to the glass, leaving the plate free at the center. Both sides of the o-ring should be covered with silicon caulk or similar stuff.

When seal and tube are ready, slowly work a small hole through the plate using a sharp gimlet. As soon as you hear air rushing in, quickly press the tube with the open end onto the rubber seal and hold in place for a few minutes. After that, the atmospheric pressure should hold it. Use more silicon to attach the tube firmly to the CRT glass.

Discharge tube attached to CRT. There are two electrodes, the big one to the right and another, smaller one (not visible) near the left end of the tube. The tube is already evacuated, but no voltage present.

Now you can start to experiment, and you should problably, as the tube might not be sealed too well and draw air slowly. With a few kV between the electrodes (or one electrode and one of the pins of the CRT socket), a nice purple glow discharge should develop inside the tube. Be careful not to overheat the tube with too much current. The best vacuum achievable with this method is in the order of 1 Torr or 1mBar, depending on the volume of the discharge tube compared to the CRT.

Glow discharge.

Larger version
The CRT is under vacuum and may implode. Be careful not to damage the glass, not even make a small scratch. Wear safety googles, protect your body with thick clothing, especially hands and face.

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