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Friday 19 August 2011

Crookes Tube - early cathode ray device

Way back when I was still at school, I took a trip to Europe. Whilst browsing Portobello Road in London, I came across a bloke selling old Geissler and Crookes tubes. I was amazed by these antique electrical devices.

A Geissler tube is something like an old neon sign tube, but uses higher voltage and gives much more interesting colours. They glow depending on the materials within.

A Crookes tube is an evacuated glass tube used for various experiments on cathode rays. By running a high voltage through them, they showed various effects and were used to prove some fundamental properties of subatomic particles, specifically the electron.

With my hard earned pocket money, I splashed out on a magnetic deflection tube. This is a long tube with an anode and cathode at each end, and a phosphor coated screen within. A slit is used to create a narrow beam of cathode rays.

In normal operation, a glowing strip is seen across the screen, from the cathode rays striking the screen. By applying a magnet to the tube, the rays (and hence the glowing strip) are deflected perpendicular to the field. This proved that electrons are emitted by the cathode (hence the term "cathode rays"), and that they are deflected by a magnetic field.

The tube in these pictures and video was bought for something like GBP30. Nowadays you'd be hard pressed to find one for under $400. It's fed from a TV flyback transformer at about 20kV. Occasional flashes can be seen as the vacuum is not perfect, and remaining gas in the tube sometimes ionises to a complete path.


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