museum items
TEL 523 cathode ray on
Magnet brought up to screen
Cross disconnected from anode
Screen coil on

Maltese-Cross Cathode Ray tube

The Maltese-Cross was one of the first shapes put in the path of 'cathode rays' to demonstrate shading on a fluorescing screen. Unlike the original tubes this one uses a heated cathode. This can be set to emit enough visible light to cast an optical shadow of the cross on the screen. We now know that Cathode-Rays are electrons emitted from the cathode. Hot cathodes emit electrons easily. This means lower Anode voltages can produce electron images.

'Mouse-over' the image to see the 'cathode-ray' image.

The reflection of the hot cathode filament can be seen on the Anode in the neck of the tube. The fluorescence glows green when hit by electrons.

This tube is from a range of cathode ray tubes developed for teaching Physics in schools and colleges.

The three fluorescent screen images below are of the Maltese-cross shadow under electron bombardment. Three different effects are shown. These can be seen by moving the 'mouse over' the image. The circuit diagrams on the right illustrate the difference in each case.

1. A strong Magnet is brought close to the top of the screen. Note how this distorts the image as the electrons interact with the magnetic field.

2. The Anode connection to the cross has been removed allowing the cross to charge up negatively. This produces a '4-leaf clover' image. The Maltese-cross becomes negatively charged with electron bombardment. This then repels other approaching electrons resulting in the image ballooning.

3. A 9V DC supply is applied to a coil placed around the front of the screen. This technique is used in some cathode ray oscilloscopes for trace rotation.

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