Crookes tube
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 The low voltage power supply (A) is connected to heat the cathode (C). The higher voltage power supply (B) energizes the phosphor-coated anode (P). The mask (M) is connected to the cathode potential and its image is seen on the phosphor as a non-glowing area. This tube may be built without supply A by using a cold cathode  |
The Crookes tube is an evacuated glass cone with 3 node elements (one anode and two cathodes). It is an invention of the 19th century scientist William Crookes and is an evolutionary development of the earlier Geissler tube.
[edit] Description of ultilization
The Crookes tube was designed to explore phosphorescent effects observed in the Geissler tubes — at high vacuum any phosphorescent material within the low pressure envelope would glow, but only at one terminal. The excitation of the phosphor was attributed by Crookes to what he called cathode rays, and which are now interpreted to not be rays but rather particles. These particles are electrons.
At the small end of a glass cone, an electrically heated wire, called the cathode, produces electrons. At the opposite end, a phosphor coated screen forming an anode is connected to the positive terminal of a voltage source of modest voltage (a few hundred volts), whose negative end is connected to the cathode. In between the cathode and the anode is placed a third element, a flat plate with some distinctive shape (the shadow mask), also connected to the cathode. When the appropriate voltages are applied to the various elements the screen will be seen to glow. A non-glowing image of the shadow mask will be imposed upon the screen.
[edit] Improvements and innovations
Replacing the shadow mask with a beam focusing cylinder created the electron gun, which enabled the creation of a spot of illumination. The addition of electrostatic deflectors in the horizontal and vertical planes allowed the visualization of voltages applied to the deflectors. In this way, the tube was eventually developed as the display portion of the oscilloscope (a diagnostic and display device for use in electronics). This was further developed as a display device for radar and television displays using external magnetic coils for focusing and deflection. The principle of the shadow mask was exploited in displays for color television sets.
It was also observed that the application of high voltage (e.g. 25,000 volts) to the anode within a high vacuum tube would produce X-rays (observed by the inadvertent fogging of photographic film, even though it was within a light proof enclosure). Specialized tubes were developed that replaced the phosphor with a metal target and which focused the beam on a small spot to aid in obtaining a point source of the rays - this would then produce a sharp image of intervening material on the film.
