From the accidental discovery that selenium was sensitive to light has developed a wide range of instruments and equipment which the engineers can use to assist him in his work
THE TELEVISION CAMERA used for exterior scenes at Alexandra Palace, London, is the Marconi E.M.I. Instantaneous Television Camera. The principle of the photoelectric or light-sensitive cell used in this camera is a development of the principle used by Senlecq in his attempt at television as early as 1879.
SCIENTIFIC progress is generally regarded as being synonymous with steady, logical, ordered evolution. A study of the history of any particular branch, however, reveals that luck has played an extraordinarily large part. Accident gives the research worker a start and from that point his progress may be deliberate and logical.
Willoughby Smith in 1873 discovered the light-sensitive properties of selenium when he was investigating its properties with an entirely different object. Because of its high resistance, the selenium was being used as an orthodox resistance in telegraph receivers. Smith’s assistant, May, in the course of these totally different experiments, stumbled on the discovery that the element was affected by light.
The effect was made manifest by seemingly inexplicable fluctuations in the current flowing in a circuit in which there were no known variable factors. The fluctuations were then found to be linked up with the position of the equipment in relation to the room illumination. The effect was traced to the selenium and it was definitely established that the value of its resistance in the light was considerably less than in the dark.
Willoughby Smith’s communication to the Institution of Electrical Engineers (then the Society of Telegraph Engineers), announcing the discovery was couched in terms fitting to the dramatic nature of the phenomenon. Only a short time before Preece had informed the Society of the results of his microphone experiments, by which the walking of a fly had been made to sound similarly to “the tramp of a horse over a wooden bridge”. Quoting this simile of Preece’s, Smith added that this feat was surpassed by his own discovery. He added the words, “With the aid of the telephone I have heard a ray of light fall on a bar of metal”.
The knowledge that selenium was sensitive to light released a flood of invention. Innumerable schemes, including one for a television system, had been suggested, but were sterile solely because some means of converting light into an electrical current was necessary for their success. Selenium apparently provided that means, and within two years a system for the transmission of pictures by wire was developed. In 1880 Graham Bell produced his “photophone”, by which intelligible speech was transmitted over a beam of light. These pioneers had extremely little information on which to build up their inventions incorporating selenium. Beyond the fact that it was in some measure responsive to light, no definite data were available. Many early attempts to make use of the effect were ambitious but failed because they presumed too far on the possibilities of the element. Research was started by several independent workers to determine more exactly the practical limits of the phenomenon.
Selenium exists in two forms, vitreous and crystalline. In its crystalline form only will it conduct electricity. It was further discovered that the element was primarily sensitive to light in the yellow and red portions of the spectrum. Exposure to bright sunlight causes its resistance to fall to a level approximating to one-tenth of its dark value.
Attention was now turned to discovering how rapidly it would respond to light changes. Its response proved to be fairly rapid, probably in excess of 5,000 cycles a second, as was indicated by the practical success of Bell’s photophone.
If the original discovery of the light-sensitive properties of selenium can be likened to the bursting of a bombshell, the subsequent effects on the scientific world sustain the analogy. With this fundamental knowledge as the centre point, research radiated in all directions. The quest for further knowledge of the element not only embraced the points already considered, but also penetrated all possible spheres of application. Selenium was tried as one of the electrodes of a voltaic cell by Sabine, in 1878, who showed that when a cell of this type was exposed to light an electromotive force was generated.
Sabine’s was an important discovery, although it was not exploited to a great extent. His was the only true form of selenium “cell”, although the term is invariably applied to what are selenium bridges. The most efficient method of using the light-sensitive properties of selenium is to spread the element in a melted state over a conducting surface.
Wire is sometimes wound over an insulating former (or shape) and the selenium arranged so as to bridge the gap between adjacent windings. Other cells are built up by interleaving copper and mica strips in sandwich fashion and by applying selenium to the edge of the sandwich. The basic idea is to make conduction possible only through the medium of the selenium, while keeping to a minimum the amount of selenium incorporated.
With cells of the bridge type, Senlecq attempted a crude form of television in 1879. His scheme had the merit of ingenuity, although it was somewhat impracticable. He tackled the problem with the idea of imitating the human eye. It was realized that the retina comprises what may be regarded as a mosaic of light-sensitive cells. Senlecq’s artificial retina was a battery of selenium cells. Each cell was connected to one shutter of a lamp screen by a pair of wires and the image to be transmitted was projected on to the cells.
According to the light and shade in the picture, each cell receives a certain amount of illumination. Depending upon the amount of light reaching any individual cell, the current passing through it will have a certain value. For a great amount of light, the resistance will be much lower and the current proportionally higher, and the reverse also holds good.
Early Picture Transmission
The cells were connected to shutters, the current through a cell being arranged to control the movement of one shutter. A cell receiving a great deal of light would cause its shutter to open wide. Behind the checkerboard of shutters was a bank of lamps, so that the extent to which a shutter opened governed the amount of light which passed through. Now the principle of the scheme is clear. The lamp screen, by the opening of the shutters, could reproduce the image originally projected on to the selenium cells. The system was complicated and crude, yet modern television has reverted to the same basic principle in the form of Dr. Zworykin’s “ Iconoscope”, used in exterior television transmissions.
In contrast to television, picture telegraphy (transmission of pictures by land line or radio) need not be instantaneous. The picture may gradually be built up at the receiving end. A nebulous form of picture telegraphy was demonstrated by Shelford Bidwell before the Physical Society in 1881. In this apparatus a beam of light was focused on a selenium cell placed in series with a battery. The picture to be transmitted was in the form of a transparency and was interposed between the cell and the light source.
Less light passed through the dark portions of the picture than through a clear section, the current through the cell varying to correspond. These current variations were passed on to a platinum pointer at the receiving end, which was arranged to move over paper treated with potassium iodide. By the liberation of free iodine, the paper was discoloured in direct proportion to the received current.
At the transmitter the picture was caused to move so that it was virtually “scanned” by the cell. Movement of the pointer at the receiver was synchronized with this movement of the picture. Thus, according to the light and shade in the picture, a given amount of current reached the platinum pointer, causing discoloration of the paper in direct proportion to the transmitted image. The pictures received in this manner were crude, but Bidwell had discovered the right principle and a fundamentally similar arrangement is used to-day.
AUTOMATIC PRINTING of photographic negatives is made possible by the aid of the photoelectric cell. The photoelectric printer, as the machine is called, automatically ensures perfect prints regardless of the character of the negative. THE PHOTOELECTRIC CELL (extreme left) contains a nickel anode and a cathode of caesium or other light-sensitive element.
In the later years of the nineteenth and in the first years of the twentieth century, research indicated that selenium had several serious defects. These became more apparent as the methods of applying the element developed from their earlier crudity. Most important of these defects was a time lag.
Selenium would respond instantaneously to light, but when the source of illumination was removed, the element did not immediately return to its former state. The delay is probably about one six-thousandth part of a second. Although this is apparently an absurdly small period, further consideration shows that it is sufficient to make a selenium cell unsuitable for talking pictures (10,000 cycles to 12,000 cycles a second) and hopelessly inadequate for modern television (up to 2,000,000 cycles a second).
Another factor which militated against the further development of selenium was the discovery of other light-sensitive elements. Knowledge of the peculiar properties of selenium undoubtedly promoted the later successful search for substances of similar characteristics, so that credit is indirectly due to Willoughby Smith. The two men who achieved most in this field were Elster and Geitel, who found that all the alkali metals such as sodium, potassium, rubidium and caesium were sensitive to light. Their work gave rise to the modern photoelectric cell, on which television, talking films and a thousand other important developments depend for their success.
Although all are similar in being affected by light, the alkali metals exhibit strikingly different characteristics from selenium in the manner in which the effect takes place.
No completely satisfactory explanation has yet been evolved for the change in resistance in selenium when exposed to light. The effect is merely that its resistance falls and nothing else happens. There might be an internal emission of electrons, but it is not certain. With the alkali metals, on the other hand, the influence of light causes particles of negative electricity (electrons) completely to leave the material.
The early forms of photoelectric cell making use of this effect consisted of a glass bulb on the inside walls of which potassium was deposited by distillation. A ring of nickel was suspended in the bulb to serve as the anode. Connexions were taken from the potassium and also from the nickel ring to the exterior. When a positive voltage was applied to the ring and the potassium was illuminated, current flowed from the potassium to the ring. To forestall any possible misapprehension, it should, perhaps, be pointed out that if selenium were used in place of the potassium, everything else remaining exactly as before, there would be no electron flow. This is the widest gulf between the two phenomena. Tests conducted to discover the rapidity with which the photoelectric cell would respond to light changes indicated that the limit was so high as to be beyond the range of measurement. Even now the limit has not been determined and appears to be set only by the transit time of the electrons. The intensity of the illumination may be changed four or five million times in a second and the photoelectric cell will not miss one change.
In the face of such extremely fierce competition, the selenium cell has lost ground, although it is still widely used when a good high-frequency response is not essential. Selenium is far more sensitive to weak illumination than the photo-cell and would certainly not have lost favour if the time-lag problem had been capable of solution.
From the time of the initial discovery by Smith and May, nearly forty years were spent in evolving the photoelectric cell. In the ensuing twenty years progress was steady and in the last decade the applications have multiplied with remarkable rapidity. Some of these applications are dealt with in the chapter on “Harnessing Light” beginning on page 387. In most instances described there the cell is used as a “light switch”, that is to say, it performs a certain function when a beam of light focused on it is broken. The cell can be used in circumstances where it must exercise more intelligence, as in the estimation of heat values.
A PHOTOELECTRIC EXPOSURE METER is incorporated in the automatic photoelectric printer so that, while one negative in a strip is being printed, the correct exposure necessary for the next is recorded automatically according to the density of the negative. The machine also automatically selects the type of paper required for a good print of any
Heat is always accompanied by some form of light radiation. At low temperatures, the radiation is not visible to the eye, but is capable of detection by a photo-cell. Similarly, as the temperature increases the character of the radiation changes until the red and white hot stages are reached. Through all these changes a photo-cell suitably arranged would record variations in the value of the transmitted light. This is the basis of the photoelectric pyrometer, in which application the cell is particularly valuable because of its amazing sensitivity. If the radiation from a piece of metal which was heated to dull red (1,400° Fahrenheit) were focused on to a photo-cell a given amount of current would flow. Increasing the temperature of the metal by 300° to medium red-heat causes the value of the current delivered by the cell to be multiplied ten times.
In many branches of industry the accurate control of temperature is of paramount importance. In steel rolling-mill operation a careful watch must be maintained on the heating of the material. An instrument has been developed, known as the “Instagraph”, which enables an instantaneous measurement of temperature to be obtained even while the steel is in motion through the rolling mill. Further, the device is arranged to provide a complete record of the temperature changes in the form of a chart.
In appearance the “Instagraph” resembles a small gun. To discover the temperature of a hot body, it is necessary merely to point the muzzle at it, the photo-cell and associated equipment immediately providing the answer. Allowance is made in the design for the exclusion of all extraneous light, and special filters ensure that radiation falling only within a desired region of the spectrum should reach the cell.
Some factories make use of elaborate conveyer systems, with several subsidiary belts branching off from the primary conveyer. Here it may be necessary for certain components on the main belt to be shunted on to these minor lines. In addition to counting packages passing along the lines, the photoelectric cell can act also as a selector.
The method adopted is to insert little signal flags in the packages according to their destinations. The spacing of the flags is such that they interrupt (or fail to interrupt) the light beams focused on the cell. Interruption of the light ray causes a trip mechanism to come into operation, so that the packages are shunted in the desired manner.
A similar idea is used for the sorting of mail bags, but for this work the cell is arranged to move along with the conveyer. Stationary light sources of varying height are mounted alongside the belt so that the cell is illuminated only when it reaches a lamp mounted at the same height as itself. Each bag is required to be dumped at any given one of a number of stations. The height of the cell is therefore adjusted by the operator to a figure corresponding with that of a lamp at the station where a particular bag is to be dumped. Cell and bag travel along together until one of these selected stations is reached. At this stage the cell receives a pulse of light and immediately arranges for the bag to be pushed off the conveyer.
The tyro as well as the experienced photographer has benefited by the advent of the photo-cell. One of the most difficult tasks in photography is accurately to estimate the value of illumination so that the length of exposure required may be determined. The photoelectric exposure meter is a straightforward application of general principles. More recently, however, the cell has still further usurped the functions of the photographer in the form of the photoelectric printer.
This machine has a formidable appearance, but its operation is quite simple, and it reduces the work entailed in the developing and printing of photographs to mere button-pressing. The machine automatically ensures perfect prints regardless of the character of the negative. It also undertakes the tasks of selecting the correct grade of printing paper and makes the correct exposure for any type of negative. Variations in the speed, make or grade of batches of printing paper are of no consequence, as the machine makes due allowance for all these points. Having performed all these tasks, the device completes the process by automatically delivering the prints down a chute to the developer.
Almost over the whole range of human endeavour the light-sensitive cell offers its services as a means of improving existing practice or opening new lines of development. The limit of its activities is set only by the ingenuity of the engineers seeking to apply it.
PHOTOELECTRIC MEASUREMENT OF TEMPERATURE is provided by the “Instagraph” outfit. To discover the temperature of a metal, for instance steel in a rolling mill, the muzzle of the instrument, resembling a gun, is pointed at the hot metal. The photoelectric cell measures the amount of radiation transmitted from the hot metal and the reading is instantaneously indicated. In addition, a complete record of all temperature changes is shown on a chart.