100 years ago it was merely a scientists dream. 70 years ago people such as Zworkyn and John Logie Baird proved the basics possible. 50 years ago owned only by the wealthy, they began to change the world. Today almost every household in Europe has at least one, they are used for entertainment, information and education. This report aims to describe how a standard Television works and what understanding of Physics was needed to develop it. (1) Background
Unlike modern Television sets the earliest were almost completely mechanical. The dream that was Television was a machine that could reproduce captured images using light, unlike photographs and film Television would store pictures electronically. The original mechanical Televisions could achieve this but were not alike Televisions based on the Cathode Ray tube (see next page). It is not surprising that the inventor of Television is greatly debated, there are claims of it being a Scotsman, a Russian and the Japanese.
It is also a matter of opinion, a Russian called Zworkyn is accredited as inventing the first electrical Television, whilst John Logie Baird is accredited as the inventor of the first commercially possible mechanical Television. It also seems to depend on National opinion. One American Physicist and Historian explained how American companies invented the Television with “People from foreign countries contributed a little here and a little there over the years”.
When this American was talking about Television it seems he was talking about the commercial Television developed for broadcasting in America from 1939 but does not explain this clearly, however he does mention how the British Broadcasting corporation introduced Television broadcasting in 1936, 3 years before America. Sources claiming it was a Scotsman include www. Kinema. com, Dorling Kindersley Science Encyclopedia The definition of Television from the Oxford Dictionary is “A system for reproducing on screen visual images transmitted by radio signals”
The scanning method of mechanical Televisions used a disk patented by a Russian called Nipkow as early as 1884. The Disk he proposed was cut with 18 holes in a spiral pattern and was capable of scanning a scene in horizontal lines. The idea was that a photoelectric cell placed behind the disk would record the light intensity that was received through each hole by translating the light intensity into a voltage. A similar disk facing a screen, with a light bulb instead of a photoelectric cell would reproduce the picture by varying in intensity at a synchronised time to the original disk.
This same idea was used in John Logie Bairds Television except this time the technology was more advanced, the signal could be transmitted though radio waves. (Example: see (1)). Nipkows machine was only capable of 4000 dots every second. Although John Logie Baird became famous his Television was not used widely. Instead electronic recording and displaying methods were used. Zworkyn, was a Russian who travelled to America to develop his Television system. Eventually his work produced a camera and display that seem very alike today’s Television.
His camera called the Iconoscope, stored light intensity electronically before displaying on the Cathode Ray Tube, which he developed for Television. The Iconoscope used either a metal conductor covered with a sheet of silver caesium or aluminium film coated with potassium Hydride, unknown because I found conflicting sources, it is likely one was an earlier model. Both Silver Caesium and Potassium Hydride are photosensitive, when they are exposed to light electrons are lost in a number proportional to the intensity of light falling on them.
The photosensitive dots would then have become positive, the charge was detected by an electron gun, which scanned the material. The picture was then shown using a Cathode Ray tube, which Zworkin pioneered. The equipment Zworkyn introduced was preparation for commercial Television. All Televisions rely upon the detection of light intensity, only possible through the discovery and understanding of the photoelectric effect. This is the effect that light has on substances, causing them to lose electrons to their surroundings.
When it was discovered the intensity of light was proven to compare directly with the number of electrons being released, the greater the intensity: the greater the number of electrons released. Also discovered was the effect of different light on the emission of electrons. Physicists at the time (late 19th Century) could not explain this, this was before the Quantum theory had been proposed. The only light theory was an early wave theory, which would not have explained why different light frequencies had different effects upon the release of electrons.
The classic wave theory models light as having all the characteristics of a wave, believing the only differences between blue light and red light to be frequency and wavelength. This theory would have explained the photoelectric effect by energy given to electrons by oscillating. The findings would suggest that on certain substances a low intensity of high frequency light would release electrons that could not be released by a large intensity of low frequency light.
Einstein developed Plancks theory that when energy was absorbed it turned into Quanta into the modern quantum theory, 18 years after the discovery of the photoelectric effect by Hertz. Einstein said that light is made from very small particles of energy, which he named quanta, that travel through the air arriving randomly, meaning that their arrival was not constant. The quantum theory and Plancks relationship between energy and frequency explained the photoelectric. Einstein said that one photon, no more, releases an electron from material.
The photons will not be able to release an electron unless they have enough energy to release an electron from a material, called the activation energy. The picture above shows a basic Cathode Ray Tube. A, the cathode is the source of the electrons, which are responsible for the overall picture. C, represents an anode, responsible for directing and accelerating the electrons. There can be up to 6 of these in a television. The rear of the screen is coated in small phosphor dots which the electron beams are aimed towards creating colour on the front of the screen. The cathode is the negative terminal in a circuit.
Its use in the Cathode Ray tube is similar to its use in electrolysis where the cathode releases electrons, which go through a molten substance, acting as a conductor, to the positive anode. This completes a circuit. In the Cathode Ray tube rather than a molten substance there is a vacuum. This allows electrons to pass through which would not happen air as the electrons would collide with molecules. Colour Television is only possible as the three primary colours of light can be mixed in varying proportions to create almost any other colour. These primary colours are red, blue and green.
An electron gun is the part of the Cathode ray tube containing the cathode and the anodes. Colour Television needs three electron beams while a black and white Television only needs one, but this does not mean three electron guns are needed (see developments of colour TV). Black and white Televisions use one beam as they only send a set intensity of electrons to each pixel, per pass. intensity set intensity of electrons to one pixel at a certain time. In a colour Television there are three beams as three different, controlled, intensities of electrons are needed at any one time, one beam for each pixel.