Williams tube
Encyclopedia
Frederic Calland Williams
Sir Frederic Calland Williams CBE, FRS , known as 'Freddie Williams', was an English engineer....
and Tom Kilburn
Tom Kilburn
Tom Kilburn CBE, FRS was an English engineer. With Freddie Williams he worked on the Williams Tube and the world's first stored-program computer, the Small-Scale Experimental Machine , while working at the University of Manchester.-Computer engineering:Kilburn was born in Dewsbury, Yorkshire and...
), developed in about 1946 or 1947, was a cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
used to electronically store binary data.
It was the first random-access
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...
digital storage device, and was used successfully in several early computers.
Working principle
The Williams tube depends on an effect called secondary emissionSecondary emission
Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particles like electrons or ions. If the secondary...
. When a dot is drawn on a cathode ray tube, the area of the dot becomes slightly positively charged and the area immediately around it becomes slightly negatively charged, creating a charge well
Potential well
A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy because it is captured in the local minimum of a potential well...
. The charge well remains on the surface of the tube for a fraction of a second, allowing the device to act as a computer memory. The lifetime of the charge well depends on the electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of the inside of the tube.
The dot can be erased by drawing a second dot immediately next to the first one, thus filling the charge well. Most systems did this by drawing a short dash
Dash
A dash is one of several kinds of punctuation mark. Dashes appear similar to hyphens, but differ from them primarily in length, and serve different functions. The most common versions of the dash are the en dash and the em dash .-Common dashes:...
starting at the dot position, so that the extension of the dash erased the charge initially stored at the starting point.
The computer reads information from the tube by means of a metal pickup-plate that covers the front of the tube. Each time a dot is created or erased, the change in electrical charge induces a voltage pulse in the pickup-plate. Since the computer knows which location on the screen is being targeted at that instant, it can use the voltage pulse from the plate to read the data stored on the screen.
Reading a memory location creates a new charge well, destroying the original contents of that location, and so any read has to be followed by a write to reinstate the original data. Since the charge gradually leaked away, it was necessary to scan the tube periodically and rewrite every dot (similar to the memory refresh
Memory refresh
Memory refresh is the process of periodically reading information from an area of computer memory, and immediately rewriting the read information to the same area with no modifications. Each memory refresh cycle refreshes a succeeding area of memory. Memory refresh is most often associated with...
cycles of DRAM
Dynamic random access memory
Dynamic random-access memory is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1...
in modern systems).
Some Williams tubes were made from radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
-type cathode ray tubes with a phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
coating that made the data visible to the eye, while other tubes were purpose-built without such a coating. The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators since the face of the tube was covered by the pickup-plate. If a visible output was needed, a second tube with a phosphor coating was used as a display device.
Each Williams tube could store about 512–1024 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s of data.
Development
Developed at the University of ManchesterVictoria University of Manchester
The Victoria University of Manchester was a university in Manchester, England. On 1 October 2004 it merged with the University of Manchester Institute of Science and Technology to form a new entity, "The University of Manchester".-1851 - 1951:The University was founded in 1851 as Owens College,...
in England, it provided the medium on which the first electronically stored-memory program was stored in the Manchester Small-Scale Experimental Machine (SSEM) computer. Tom Kilburn wrote a 17-line program to calculate the highest factor of 218. Tradition at university has it that this was the only program Kilburn ever wrote.
In fact, rather than the Williams tube memory being designed for the SSEM, the SSEM was a testbed to demonstrate the reliability of the memory.
The Williams tube tended to become unreliable with age, and most working installations had to be "tuned" by hand. By contrast, mercury delay line memory
Delay line memory
Delay line memory was a form of computer memory used on some of the earliest digital computers. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory, but as opposed to modern random-access memory, delay line memory was serial-access...
was slower and also needed hand tuning, but it did not age as badly and enjoyed some success in early digital electronic computing despite its data rate, weight, cost, thermal and toxicity problems. However, the Manchester Mark 1 was successfully commercialised as the Ferranti Mark 1. Some early computers in the USA also used the Williams tube, including the IAS machine
IAS machine
The IAS machine was the first electronic computer built by the Institute for Advanced Study , in Princeton, New Jersey, USA. It is sometimes called the von Neuman machine, since the paper describing its design was edited by John von Neumann, a mathematics professor at both Princeton University...
(originally designed for Selectron tube
Selectron tube
The Selectron was an early form of digital computer memory developed by Jan A. Rajchman and his group at the Radio Corporation of America under the direction of Vladimir Zworykin, of television technology fame...
memory), the UNIVAC 1103
UNIVAC 1103
The UNIVAC 1103 or ERA 1103, a successor to the UNIVAC 1101, was a computer system designed by Engineering Research Associates and built by the Remington Rand corporation in October, 1953...
, Whirlwind
Whirlwind (computer)
The Whirlwind computer was developed at the Massachusetts Institute of Technology. It is the first computer that operated in real time, used video displays for output, and the first that was not simply an electronic replacement of older mechanical systems...
, IBM 701
IBM 701
The IBM 701, known as the Defense Calculator while in development, was announced to the public on April 29, 1952, and was IBM’s first commercial scientific computer...
, IBM 702
IBM 702
The IBM 702 was IBM's response to the UNIVAC—the first mainframe computer using magnetic tapes. Because these machines had less computational power than the IBM 701 and ERA 1103, which were favored for scientific computing, the 702 was aimed at business computing.The system used electrostatic...
and the Standards Western Automatic Computer
SWAC (computer)
The SWAC was an early electronic digital computer built in 1950 by the U.S. National Bureau of Standards in Los Angeles, California. It was designed by Harry Huskey...
(SWAC). Williams tubes were also used in the Soviet Strela-1
Strela computer
Strela computer was the first mainframe computer manufactured serially in the Soviet Union, beginning in 1953.This first-generation computer had 6200 vacuum tubes and 60,000 semiconductor diodes....
.
Alternatives
Alternatives to the Williams Tube included delay line memoryDelay line memory
Delay line memory was a form of computer memory used on some of the earliest digital computers. Like many modern forms of electronic computer memory, delay line memory was a refreshable memory, but as opposed to modern random-access memory, delay line memory was serial-access...
and Selectron tube
Selectron tube
The Selectron was an early form of digital computer memory developed by Jan A. Rajchman and his group at the Radio Corporation of America under the direction of Vladimir Zworykin, of television technology fame...
.
See also
- Regenerative capacitor memoryRegenerative capacitor memoryRegenerative capacitor memory is a type of computer memory that uses the electrical property of capacitance to store the bits of data. Because the stored charge slowly leaks away, these memories must be periodically regenerated Regenerative capacitor memory is a type of computer memory that uses...
- Mellon optical memoryMellon optical memoryMellon optical memory was an early form of computer memory invented at the Mellon Institute in the 1950s. The device used a combination of photoemissive and phosphorescent materials to produce a "light loop" between two surfaces. The presence or lack of light, detected by a photocell, represented a...