Part 6 (1/2)
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.]