P, P", plates; M, mica; S, selenium.]
[Ill.u.s.tration: FIG. 53a.]
A strong light falling upon a cell lowers its resistance, and _vice versa_, the resistance of a cell being at its highest when unexposed to light; the light is apparently absorbed and made to do work by varying the electrical resistance of the selenium. Selenium cells vary very considerably as regards their quality as well as in their electrical resistance, it being possible to obtain cells of the same size for any resistance between 10 and 1,000,000 ohms, and also, a cell may remain in good working condition for several months, while another will become useless in as many weeks.
The ability of a cell to respond to very rapid changes in the illumination to which it is exposed is determined largely upon its inertia, it being taken as a general rule {111} that the higher the resistance of a cell the less the inertia, and _vice versa_, and also, that the higher the resistance the greater the ratio of sensitiveness. Inertia plays an important part in the working of a cell, slightly opposing the drop in resistance when illuminated, and opposing to a [Ill.u.s.tration] much greater degree the return to normal for no-illumination. The effects of inertia or "lag," as it is termed, can readily be seen by reference to Fig. 55. It will be noticed that the current value rapidly increases when the cell is first illuminated, but if after a short time _t_ the light is cut off, the current value, instead of returning at once to normal for no-illumination, only partially rises owing to the interference of the inertia, and some time elapses before the cell returns to its normal condition; the time varying from a few seconds to several minutes, depending upon the characteristics of the cell and the amount of light to which it is exposed.
An actual curve is given in Fig. 55a. The inertia or "lag" of a cell produces upon an intermittent current an effect similar to that produced by the capacity [Ill.u.s.tration] of a line, as was noted in Chapter I., preventing the incoming signals from being recorded separately, and distinctly. To obtain the best results in photo-telegraphy, the resistance of a cell should only be decreased to an extent sufficient to pa.s.s the current required to operate the recording apparatus, and the illumination should be regulated so that this condition of the cell takes place.
The comparative slowness of selenium in responding to {112} 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 (see Chapter I.) 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 p.r.o.nounced. 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.
[Ill.u.s.tration: FIG. 55a.]
Light from the transmitting or receiving apparatus, as the case may be, falls upon the selenium cell S^1, which is {113} 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 (described in Chapter III.), 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.
[Ill.u.s.tration: FIG. 56.]
It is also stated that by enclosing the cells in exhausted gla.s.s tubes, their inertia can be greatly reduced and their life considerably prolonged.
The sensitiveness of a cell is the ratio between its resistance in the dark and its resistance when illuminated. The majority of cells have a ratio between 2:1 and 3:1, but Professor Korn has shown mathematically that by conforming to certain conditions regarding the construction the ratio of sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only a short distance away; the resistance may be still {114} further decreased by continuing the illumination, but this produces a permanent defect in the cells termed "fatigue," the cells becoming very sluggish in their action and their sensitiveness gradually becoming less, the ratio between their resistance in the dark and their resistance when illuminated being reduced by as much as 30 per cent.
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.
{115}
APPENDIX B
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.
As will have been gathered from Chapter II., the camera used for copying has to have a single line screen placed a certain distance in front of the photographic plate, and the object of this screen is to break the image up into parallel bands, each band varying in width according to the density of the photograph from which it has been prepared. Thus a white portion of the photograph would consist of very narrow lines wide apart, while a dark portion would be made up of wide lines close together; a black part would appear solid and show no lines at all. It is, of course, obvious {116} that the lines on the negative cannot be wider apart, centre to centre, than the lines of the screen. A good screen distance has been found to be 1 to 64, _i.e._ the diameter of the stop is 1/64th of the camera extension, and the distance of the screen lines from the photographic plate is 64 times the size of the screen opening. The following table shows what this distance is for the screen most likely to be used. The line screens used consist of gla.s.s plates upon which a number of lines are accurately ruled, the width of the lines and the s.p.a.ces between being equal; the lines are filled in with an opaque substance. These ruled screens are very expensive, and are only made to order,[10] a screen half-plate size costing from 21s. to 27s.
6d. An efficient subst.i.tute for a ruled screen can be made by taking a rather large sheet of Bristol board and ruling lines across in pure black drawing ink, the width of the lines and the s.p.a.ces between being 1/12th of an inch respectively. A photograph must be taken of this card, the reduction in size determining the number of lines to the inch. A card 20 15 inches, with 12 lines to the inch, would, if reduced to 5 4 inches, make a screen having 48 lines to the inch. Preparing the board is rather a tedious operation, but the line negative produced will be found to give results almost as good as those obtained from a purchased screen.
DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.
-------------------------------------------------------------- |Screen ruling |Actual s.p.a.ce| Distance of |In 1/32|In milli-| |lines per inch.| in inches. |screen ruling| inches| metres.| | | | in inches. | | | |---------------+------------+-------------+-------+---------| | 35 | 1/70 | .91 | 28.8 | 21.8 | | 50 | 1/100 | .64 | 20.5 | 16.2 | | 65 | 1/130 | .49 | 15.7 | 12.4 | --------------------------------------------------------------
As it is impossible for many to have the use of professional apparatus designed for this particular kind of work, {117} 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.
[Ill.u.s.tration: FIG. 57.]
The writer has never seen a half-plate box camera, but one taking a 5 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 bra.s.s about 1/2 inch wide, are bent to the shape shown (an enlarged section is given at C) 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 (the film side) and the back of the line screen (also the film side), 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.
[Ill.u.s.tration: FIG. 58.
M, sheath; P, photographic plate; D, clips; S, line screen.]
In all probability there will be enough clearance between the top of the sheath and the top of the camera to allow for the thickness of the clip, but if not, a shallow groove a little wider than the clip should be carefully cut in the top of the camera, so that it will slide in easily.
The screen should be placed between the clips, the film side on the {118} inside, _i.e._ facing the photographic plate. As with a box camera the extension is a fixture, the size of stop to be used is a fixture also. The extension of a camera (this term really applies to a bellows camera) is measured from the front of the photographic plate to the diaphragm, and if this distance in our camera is 8 inches, then the diameter of the stop to give the best results would be 1/64th of this, or 1/8th inch. Although for all ordinary experimental work the lens fitted to the camera will be suitable, the best type of lens for process work of all kinds is the "Anastigmat."
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 {119} [Ill.u.s.tration] [Ill.u.s.tration] 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 pa.s.sed 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 gla.s.s 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 {120} 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 (a flat hog"s-hair as wide as possible) 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 {121} plates, should be pa.s.sed 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 bichromate should be dissolved in the water, and, when added to the glue, stir very thoroughly in order that complete mixing may take place.
The coating may be done in a good light, not bright sunlight, but _it must be dried in the dark_, because, although insensitive while in a moist condition, it becomes sensitive immediately on desiccation. If allowed to dry in the light the whole coating will become insoluble, and for this reason the brushes used should be washed out as soon as they are finished with. The sheets will take about 15 minutes to dry in a perfectly dry room, but it is not advisable to prepare many sheets at once, as they will not keep for more than two or three days.
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 gla.s.s 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 {122} 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 s.p.a.ces 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 {123} 20 minutes, as the amount of light which falls upon an object decreases as the inverse square of the distance.