![]() ![]() The speed of the electrons can be calculated from an energy balance: 0.5mv 2 = eV, or v = √2eV/m. The electron current is a few microamperes. ![]() By adjusting this voltage, the focal length of the equivalent electron lens is changed. The electrons are accelerated by a second anode at high potential, more than 500 V in most cases, which is in two parts, separated by a first anode at a somewhat lower voltage. The intensity is regulated by the negative potential on a thimble in front of the cathode that has a small hole which is the approximate point source of electrons. The source of electrons is a thermionic cathode. The electron gun produces a beam of high-speed electrons focussed to a small spot on the screen. The fluorescent and phosphorescent light are usually different colors. The light emitted at or soon after electron impact is fluorescence, while delayed light is phosphorescence. The "phosphor" is a luminescent material that emits light when excited by energetic electrons. We will mainly be concerned here with the oscilloscope display tube, which has electrostatic focusing and deflection, and a monochromatic, short persistence phosphor. The common picture tube is a CRT with magnetic focussing and deflection, often containing three electron beams that are directed to separate phosphor spots to display different colors, and which are modulated to display different intensities. It is essentially an inertialess pointer that can be moved rapidly in two dimensions, and can be modulated in intensity. The Cathode-ray tube (CRT) consists of an electron gun, deflecting apparatus, and luminescent screen. Polaroid photography of oscilloscope screens was popular until digital scopes, that stored a digital record of events, became available. Such displays were necessary for the radar of the 1940's, which would hardly have been possible without them. Later, electrons exposed the plates through a very thin metal window, but before long brighter luminescent screens were developed, creating the oscilloscope, on which the display could be watched visually, and even photographed with short exposures to record transient events. An oscillograph records what happens in a brief interval of time somehow, in some early devices on a photographic plate placed inside the vacuum, so the apparatus had to be pumped down for each exposure. The first high-vacuum oscillograph of the modern type was demonstrated by Zworykin in 1929. Today, we can hardly imagine an electronics laboratory without one. In spite of all this activity, the oscilloscope did not really come into widespread use until much later. In 1908, ZnS was suggested as a luminescent material by Giesel and Zenneck for brighter screens. This was an attempt to amplify analog signals that was soon superseded by triode amplifiers. The curious and almost forgotten development of cathode-ray relays by von Lieben and others around 1910 used similar tubes, as well as electrostatic deflection. Ryan, in the United States, had a 5" tube made in Germany which used magnetic deflection, with which he studied waveforms in 1903. MacGregor-Morris used a Braun-Zenneck type tube in England in 1902. Long photographic exposures were necessary to record the repetitive pattern on the dim luminescent screen. Zenneck improved Braun's tube in 1899, using a linear sweep to display waveforms. Thomson indeed used a sort of cathode-ray tube, and it was only necessary to adapt it to engineering measurements. Ferdinand Braun in Germany seems to be the pioneer, reporting his work in 1897. The idea of an oscilloscope to display transient phenomena appeared actually very early. Although CRT's are still widely used in oscilloscopes, the manufacturers seem to use tubes specially designed for them, while commercial types such as the 2AP1A are not produced, since people do not generally make their own oscilloscopes. Because of the serious high voltage, a CRT should not be made on the breadboard, but properly constructed in a permanent form. These are the two most expensive components, followed by the 5" x 10" x 3" aluminum chassis and the potentiometer controls. A CRT and a power transformer can be purchased from Antique Electronic Supply, 6221 S. Happily, it turns out to be quite possible to make an oscilloscope from scratch for about $100, which is not really a great deal these days. The CRT is still in widespread use, the most numerous survivor of the vacuum tube tribe, and has much intrinsic interest besides, so it would make a good laboratory exercise. The three parts are the scope itself, the deflection amplifiers, and the power supply for the deflection amplifiers. However, I built the oscilloscope shown at the right. The cathode-ray tube oscilloscope is an experiment that I had thought was beyond the scope of these pages, mainly because of the cost and difficulty of obtaining the components. ![]()
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