Electron Microscope

Cathode rays exist in the form of streams of high speed electrons in vacuum tubes.

The invention of the CRT paved the way for the invention of television.

The CRT was widely used in computer monitors prior to the advent of flat screens.

The electron gun consists of a heated filament, cathode, anode and grid.

The CRT needs time to "warm up". In older tubes, this could take several seconds.

The grid controls the numbers of electrons allowed to hit the screen.

The anode accelerates and focusses the electrons.

Colour TV used 3 electron guns.

To see a diode in action, view the demonstration.

The Cathode Ray Tube, CRT, was invented by Braun. In the earliest version of the CRT, the cathode was not heated, and was essentially a modified Crookes tube. The first version to use a hot cathode was developed by J. B. Johnson around 1920. The invention of the CRT paved the way for the invention of television and cathode ray tubes were used in all television sets until the late 20th century, when the plasma screen was developed.

Cathode rays exist in the form of streams of high speed electrons emitted from a hot cathode inside a vacuum tube. The electrons are liberated from the cathode by a process called thermionic emission. They are then accelerated and collimated by a potential difference (voltage) between the cathode and a second electrode called the anode. The direction of this beam is then altered either by a magnetic or electric field, or both. Consequently the beam of electrons can be made to strike various points on the inside surface of a phosphorescent screen. Light is emitted by that material at the instant that electrons hit it.

In television sets and modern computer monitors, the screen is sprayed by electrons, and a picture is created by varying the intensity of the electron beam using the received television signal. The direction of the electron beam is changed by a varying magnetic field produced by electromagnets. This is known as "magnetic deflection".

The cathode ray oscilloscope, CRO, is a dedvelopment of the cathode ray tube, CRT. Oscilloscopes use longer and narrower tubes than television (reducing the deflection angle). In the CRO, the intensity of the electron beam is kept constant, and the picture is the characteristic green “trace” on the screen. In time-base mode, the horizontal deflection is proportional to time (measured out by a "sweep oscillator" in the oscilloscope), and the vertical deflection is proportional to the input signal. The beam of electrons is deflected by applying an electric field to the deflection plates. The use of an electrical field is known as "electrostatic deflection", and it allows the electron beam to be steered more quickly than with a magnetic field.

Electron Gun

The electron gun, produces a stream of electrons by thermionic emission and then focuses it into a thin beam. The gun is located in the narrow, cylindrical part of a CRT. It consists of a heated filament, cathode, anode and grid.

1. Filament --heats the cathode with a current of a few amperes.

2. Cathode --a metal surface, which emits electrons when it is heated. A typical emission current would be a few milliamps.

3. Control grid -- controls the electron current and consequently the brightness of the image. It does so by applying a negative bias (voltage) between it and the cathode. Adjusting the “intensity” control can vary the magnitude of this reverse bias. By the time the electron beam reaches the screen, it is reduced to a few micro-amps. The spot should not be brighter than necessary as this may damage the screen.

4. Anode -- consists of a pair of open metal cylinders arranged coaxially so that the electrons pass through them along the axis. They perform a dual role of accelerating and focusing the beam.

Colour tubes use three different materials, which specifically emit red, green, and blue light, closely packed together in strips. When colour television was developed it used colour CRTs. These contain three electron guns (one for each primary colour) usually constructed as a single unit. These emit red, green and blue light. Each gun can reach only the dots of one colour on the phosphorescent screen. The outer glass allows the light generated by the phosphor out of the monitor or television screen. CRT displays accumulate static electrical charge on the screen, unless preventive measures are taken. The high voltage (E.H.T.) used for accelerating the electrons is provided by a transformer.

The output of the transformer is rectified and the fluctuating output voltage is smoothed by a capacitor. The CRT-based displays are now largely obsolete, because the new plasma screens are less bulky and consume less power.