Read Ebook: Wireless Transmission of Photographs Second Edition Revised and Enlarged 1919 by Martin Marcus J

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Besides the Hurter & Driffield method of obtaining the speed numbers of plates and films adopted by a large number of makers in this country, there are also two standard English systems known as the W.P. No. and Wynne F. No., both of which are used to a fair extent.

TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS

Although theoretically the higher the speed of the film the less the duration of exposure required, there is a practical limit, as besides the intensity and actinic value of the light admitted to the film a definite time is necessary for it to overcome the chemical inertia of the sensitised coating and produce a useful effect. With every make of film it is possible to give so short an exposure that although light does fall upon the film it does no work at all--in other words, we can say that for every film there is a minimum amount of light action, and anything below this is of no use. The exposure that enables the smallest amount of light action to take place is termed the limit of the smallest useful exposure.

There is also a maximum exposure in which the light affects practically all the silver in the film, and any increased light action has no increased effect. This is the limit of the greatest useful exposure.

In photo-telegraphy the duration of exposure, as already pointed out, is determined by certain conditions connected with the transmitting apparatus, and with conditions similar to those mentioned on page 75 the length of exposure will vary roughly from 1-50th to 1-150th of a second.

The most suitable film to use for purposes of photo-telegraphy is one having a fairly slow speed in which the range of exposure required comes well within the limits of the film. There is no advantage in using a film having a speed of, say, H. & D. 300 if good results can be obtained from one with a speed of, say, H. & D. 200, as the use of the higher speed increases the risk of overexposure. With the high-speeded films the difficulties of development are also greatly increased, there being more latitude in both exposure and development with the slower speeds, and consequently a better chance of obtaining a good negative.

Another point, often puzzling to the beginner, and which increases the difficulty of choosing a suitable make of film, is that, although one make of film marked H. & D. 100 will give good results, another make, also marked H. & D. 100, will give very poor results. This is owing, not to a poor quality film, as many suppose, but to the almost insurmountable difficulty of makers being able to employ exactly the same standard of light for testing purposes, so that although various makes may all be standardised by the H. & D. method, films bearing the same speed numbers may vary in their actual speed by as much as 30 to 50 per cent.

SELENIUM CELLS

Selenium is a non-metallic element, and was first discovered by Berzelius in 1817, in the deposit from sulphuric acid chambers, which still continues the source from which it is obtained for commercial purposes, although it is found to a small extent in native sulphur. Its at. wt. is 79.2, and its sp. gr. 4.8. Symbol, Se.

In its natural state selenium is practically a non-conductor of electricity, its resistance being forty thousand million times greater than copper. Its practical value lies in the property which it possesses, that when in a prepared condition it is capable of varying its electrical resistance according to the amount of light to which it is exposed, the resistance decreasing as the light increases.

The cell used for commercial purposes is usually constructed as follows. A small rectangular piece of porcelain, slate, mica, or other insulator, is wound with many turns of fine platinum wire. The wire is wound double, as shown in Fig. 54, the spaces between the turns being filled with prepared selenium. A thin glass cover is sometimes placed over the cell to protect the surface from injury.

The comparative slowness of selenium in responding to any great changes in the illumination offers a serious difficulty to its use in photo-telegraphy, but various methods have been devised whereby the effects of inertia can be counteracted. In the system of De' Bernochi the changes in the illumination are neither very rapid nor very great, and the inertia effects would therefore be very slight; but in any photo-telegraphic system in which a metal line print is used for transmitting, where the source of illumination is constant and the resistance of the cell is required to drop to a definite value and return to normal instantly, many times in succession, the inertia effects are very pronounced. The most successful method of counteracting the inertia is that adopted by Professor Korn of always keeping the cell sufficiently illuminated to overcome it, so that any additional light acts very rapidly. Another method worked out and patented by Professor Korn, and known as the "compensating cell" method, gives a practically dead beat action, the resistance returning to its normal condition as soon as the illumination ceases. The arrangement is given in the diagram Fig. 56.

Light from the transmitting or receiving apparatus, as the case may be, falls upon the selenium cell S^1, which is placed on one arm of a Wheatstone bridge, a second cell S^2 being placed on the opposite arm. The selenium cell S^1 should have great sensitiveness and small inertia, the compensating cell S^2 having proportionally small sensitiveness and large inertia. Two batteries B, B', of about 100 volts, are connected as shown, B being provided with a compensating variable resistance W; W' is also a regulating resistance. When no light is falling upon the cell S^1, light from L is prevented from reaching the second cell S^2 by a small shutter which is fastened to the strings of the Einthoven galvanometer , and the piece of apparatus C--relay or galvanometer as the case may be--remains in a normal condition. When, however, light falls upon the cell S^1, the balance of the bridge is upset, and light from L falls a fraction of a second later upon the second cell S^2, and the current flowing through C completes the circuit. Needless to say it is necessary that the two cells be well matched, as it is very easy to have over-compensation, in which case the current is brought below zero.

Excessive illumination will also produce similar results. The inertia of a cell is practically unaffected by the wavelength of the light used, but the maximum sensitiveness of a cell is towards the yellow-orange portion of the spectrum.

In addition to light, heat has also been found to vary the electrical resistance of selenium in a very remarkable manner. At 80? C. selenium is a non-conductor, but up to 210? C. the conductivity gradually increases, after which it again diminishes.

PREPARING THE METAL PRINTS

Electricians who desire to experiment in photo-telegraphy, but who have no knowledge of photography, may perhaps find the following detailed description of preparing the metal prints of some value. The would-be experimenter may feel somewhat alarmed at the amount of work entailed, but once the various operations are thoroughly grasped, and with a little patience and practice, no very great difficulty should be experienced. The simpler photographic operations, such as developing, fixing, etc., cannot be described here, and the beginner is advised to study a good text-book on the subject.

The method to be given of preparing the photographs is practically the only one available for wireless transmission, and although the manner given of preparing is perhaps not strictly professional, having been modified in order to meet the requirements of the ordinary amateur experimenter, the results obtained will be found perfectly satisfactory.

DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.

As it is impossible for many to have the use of professional apparatus designed for this particular kind of work, the fixing of the screen into an ordinary camera must be left to the ingenuity of the worker. A half-plate back focussing camera will be found suitable for general experimental work, but if this is not available, a large box camera can be pressed into service.

The writer has never seen a half-plate box camera, but one taking a 5 x 4 inch plate can be obtained second-hand very cheaply. It is a comparatively simple matter to fix the line screen into a camera of this description, the drawings Figs. 57 and 58 showing the method adopted by the writer. The two clips D, made from fairly stout brass about 1/2 inch wide, are bent to the shape shown and soldered at the top and bottom of one of the metal sheaths provided for holding the plates. The distance between the front of the photographic plate and the back of the line screen , indicated by the arrow at A, is determined by the number of lines on the screen. As will be seen from the table given, the distance for a screen having 50 lines to the inch will be 41/64ths of an inch.

The picture or photograph from which it is desired to make a print should be fastened out perfectly flat upon a board with drawing pins, and if a copying stand is not available it must be placed upright in some convenient position. The diagram Fig. 59 gives the disposition of the apparatus required for copying. A simple and inexpensive copying stand is shown in Fig. 60. The blackboard A should be about 30 inches square, and must be fastened perfectly upright upon the base-board B. The stand C should be made so that it slides without any side play between the guides D, and should be of such a height that the lens of the camera comes exactly opposite the centre of the board A. The camera, if of the box type, can be secured to the stand by means of a screw and wingnut, the screw being passed from the inside as shown. The beginner is advised to photograph only very bold and simple subjects, such as black and white drawings or enlargements. It is not safe to trust to the view-finders as to whether the whole of the picture is included on the plate, a piece of ground glass the same size as the plate sheaths, and used as a focussing screen, being much more reliable. It is a good plan to focus the camera for a number of different-sized pictures, marking the board A, and the guides D, so that adjustment is afterwards a very simple matter.

The make of plate used is also a great factor in getting a good negative, and Wratten Process Plates will be found excellent. As already mentioned, such subjects as the exposure and the development of the plate cannot be dealt with here, these subjects having been exhaustively treated in several text-books on photography. With an arc lamp the exposure is about twice as long as in daylight, but the exposure varies with the amount of light admitted to the plate, character of the source of light, and the sensitiveness of the plate used, etc. The writer has used acetylene gas lamps for this purpose with great success. The beginner is advised to use artificial light, as this can be kept perfectly even. With daylight, however, the light is constantly fluctuating, and this renders the use of an actinometer a necessity for correct exposure. After development, if the plate is required for immediate use, it can be quickly dried by soaking for a few minutes in methylated spirit.

Having obtained a good negative, the next operation is to prepare what is known as a metal print. For this we shall require some stout tin-foil or lead-foil, about 12 or 15 square feet to the pound, and this should be cut into pieces of such a size that it allows a lap of 3/16 inch when wrapped round the drum of the transmitting machine. Obtain some good fish-glue and add a saturated solution of bichromate of potash in the proportion of 4 parts of potash to 40 or 50 parts of glue. Pour a little of this glue into a shallow dish, lay a sheet of foil upon a flat board, and with a fairly stiff brush proceed to coat the sheet of foil with a thin but perfectly even coating of glue. The thickness of the coating can only be found by trial, for if the coating is too thick a longer time will be required for printing; but it must not be thin enough to show interference colours. After the coating has been laid on, a soft brush, such as photographers use for dusting dry plates, should be passed up and down, and across and across, with light, even strokes to remove any unevenness. A glue solution used by professional photo-engravers is as follows:

Fish-glue 12 oz. Bichromate of Ammonia 3/4 oz. Water 18 to 24 oz. Ammonia .880 30 minims.

The prepared negative must now be placed in an ordinary printing frame, and a print taken off upon one of the metal sheets in the same way as a print is taken off upon ordinary sensitised paper. In daylight the exposure varies from 5 to 20 minutes, but in artificial light various trials will have to be made in order to get the best results, the exposure varying with the amount of bichromate in the coating; the proportion of the bichromate to the glue should remain about 6 per cent. Light from a 25 ampere arc lamp for 2 to 5 minutes, at a distance of 18 inches, will generally suffice to "print" the impression on the metal sheets. The printing finished, the metal print should be laid upon a sheet of glass and held under a running stream of water. The washing is complete as soon as the unexposed parts of the glue coating have been entirely washed away leaving the bare metal, and this will take anything from 3 to 7 minutes, depending upon the thickness of the film. As soon as it is dry the print is ready for use.

As already mentioned, the negative from which the metal print is made requires that the lines be perfectly sharp and opaque, and the spaces between perfectly transparent. Ordinary dry plates are too rapid, a rather slow plate being required. Wratten Process Plates give excellent results, and the following is a good developer to use with them:

Glycin 15 grammes 1 oz. Sulphite of Soda 40 ,, 2-1/2 ,, Carbonate of Potash 80 ,, 5 ,, Water 1000 c.c. 60 ,,

This developer should be used for 6 minutes at a temperature of 50? F., 3-1/2 minutes at 65?, and 1-3/4 minutes at 80?. It is best only used once. If an intensifier is required, the following formula will be found to give satisfactory results:

Bichloride of Mercury 1 oz. 60 grammes. Hot Water 16 ,, 1000 c.c.

Allow to cool, completely pour off from any crystals, and add:

Hydrochloric Acid 30 minims 4 c.c.

Allow negative to bleach thoroughly, wash well in water, and blacken in 10 per cent ammonia .880, or 5 per cent sodium sulphide.

In preparing the negatives and metal prints the following points should be observed:

A good negative should have the lines perfectly sharp and opaque; there should be no "fluff" between the lines even when they are close together.

A properly exposed and developed negative should not require any reducing or intensifying.

If the lamps used for illuminating the copying board are placed 2 feet away, and the exposure required is 5 minutes, the exposure, if the lamps are placed 4 feet away, will be 20 minutes, as the amount of light which falls upon an object decreases as the inverse square of the distance.

Get the coating on the foil as thin as possible, and err on the side of over-exposure, for if the coating is thick and has been under-exposed, excessive washing will dissolve the whole coating; for, unless insolubilisation has taken place right up to the metal base, the under parts will remain in a more or less soluble condition.

On no account must the unexposed sheets be placed near a fire, otherwise they will be spoilt, the whole coating becoming insoluble; heat acting in the same manner as light.

In washing, keep the print moving so that the stream of water does not fall continually in one place. It is best to hold the print so that the water runs off in the direction of the lines.

To dry the prints after washing they can be laid out flat in a moderately warm oven, or before a stove, the heat of course not being sufficient to cause the coating to peel.

To render the glue image more distinct the print should be immersed for a few seconds in an aniline dye solution, the glue taking up the colour readily. These dyes are soluble in either water or alcohol. A dye known as "magenta" is very good.

The process of coating the metal sheets must be performed as quickly as possible , as owing to the peculiar nature of the bichromated glue it soon sets, and once this has taken place it is impossible to smooth down any unevenness.

See that the negative and metal sheet make good contact while printing.

If the glue solution does not adhere to the surface of the foil in a perfectly even film, but assumes a streaky appearance, a little liquid ammonia, or a weak solution of nitric acid, rubbed over the surface of the foil, which is afterwards gently scoured with precipitated chalk on a tuft of cotton wool, will remove the grease which is the cause of the difficulty.

A photograph of a picture prepared from a line negative is given in Fig. 61. For a great many experiments, and in order to save time, trouble, and expense, sketches drawn upon stout lead-foil in an insulating ink will answer the purpose admirably, but if any exact work is to be done a single line print is of course absolutely necessary. The insulating ink can be prepared by dissolving shellac in methylated spirit, or ordinary gum can be used. A very fine brush should be used in place of a pen, as the gum will not flow freely from an ordinary nib unless greater pressure than the foil will safely stand be applied. A sketch prepared in this manner is shown in Fig. 62. A little aniline dye should be added to the gum to render it more visible, or a mixture of gum and liquid indian ink will be found suitable.

In reducing, a 1/4-plate lens will be found to fully cover a 5 x 4 inch plate, providing the reduction is not greater than three to one.

LENSES

In this small volume it is not desirable, neither is it intended, to give an exhaustive treatment on the subject of lenses and their action, but as optics plays an important part in the transmission of photographs, both by wireless and over ordinary conductors, the following notes relating to a few necessary principles have been included as likely to prove of interest.

All liquid and solid bodies that are sufficiently transparent to allow light rays to pass through them possess the power of bending or refracting the rays, the degree of refraction, as already explained, depending upon the nature of the body.

In considering the action of a lens we can regard any lens as being built up of a number of prisms with curved faces in contact. Such a lens is shown in Fig. 67, the light rays being refracted towards the base of the prisms or towards the normal, as already explained; while the top half of the lens will refract all the light downwards, the bottom half will act as a series of inverted prisms and refract all the light upwards.

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