[Ill.u.s.tration: FIG. 50.
M, terminals for connecting to electric clock; L, white lamp; L^1, blue lamp; L^2, red lamp.]
The lamps are coloured, the lamp L being white, and the lamps L^1 and L^2 blue and red respectively, and care must be taken in connecting up that when the needle K makes contact with the stud P the white lamp L is in circuit. When the machines are working, the operator, by means of the brake (already described), reduces the speed of the driving motor until the needle K travels in unison with the disc J, making permanent contact with P on the contact {95} block Q, which is evidenced by the lamp L remaining alight. If, however, the needle travels faster than the disc J, contact with P is broken and fresh contact is made with P^2, the lamp L is extinguished and the red lamp L^2 lights up, and remains alight until the operator reduces the speed. Similarly, too, if the needle travels slower than J, contact is made with P^1, and the circuit of the blue lamp L^1 is completed. When the speed is either above or below the normal, the needle K engages with one or the other of the pins D, and as the tension of the driving belt is only such as is required to drive the needle, the belt slips on the pulleys until the normal speed is regained.
METHOD OF WORKING
The clockwork motor M, Fig. 51, should be capable of running for several hours with one winding, and powerful enough to take up the work of driving the machine without any appreciable effort. The main spindle of the motor is so arranged that it makes one revolution in two minutes, and the reduction in speed between the motor shaft and the shaft to which the coupling A is attached is 30:1. The metal line print having been wrapped round the drum of the machine, the stylus is put into position, at the edge of the lap, and with the needle resting about half-way on {96} the margin of the bare foil left at the commencing edge of the print. Now, when the two stations are in perfect readiness for work, the motors are started and the speed adjusted; the speed of the machine being just under one revolution in four seconds.
[Ill.u.s.tration: FIG. 51.
M, clockwork motor; S, isochroniser; E, friction break; T, brushes; F, electric clutch; X, gearing; D, D^1, switches; A, flexible coupling; K, polarised relay; L, circuit breaker; B_1, B_2, B_3, batteries; P, electric clock; W, terminals for connection to telephone relay; H, terminals for connection to terminals J, on transmitting machine.]
The switch D is then closed, and the arm of the switch D^1 placed on the contact stud (1), at the transmitting station only. As soon as the switches are closed the clutch F comes into action, and the transmitting machine begins to revolve. When the whole of the line print wrapped round the drum of the machine has pa.s.sed under the stylus, the end of the shaft D, Fig.
36, engages {97} with the spring _m_, breaking the clutch circuit and allowing the motor to run free. As soon as the machine stops, the switch D is opened and the machine run back to its starting position by hand.
At the receiving station the switch D is also closed, and the arm of the switch D^1 placed on the contact stud (2). The closing of these switches does not bring the clutch F into operation until current from the telephone relay U connected to the wireless receiving apparatus works the sensitive polarised relay K, which in turn completes the circuit of the circuit-breaker L. When the armature of L is attracted, the circuit of the relay K is broken, the circuit of the clutch F is completed, and the machine starts revolving.
[Ill.u.s.tration: FIG. 52.]
The current from the relay U, due to the transmitting stylus pa.s.sing over _one_ contact strip on the metal print, is too brief to actuate the heavier mechanism of the relay K, hence the need of the margin of bare foil at the commencing edge of the metal print, so that a practically continuous current will flow to the relay K until the armature is attracted. As, however, the relay is not actuated at the receipt of the first signal, and as it is necessary for the machine to start recording at a certain point on the film, viz. {98} at the edge of the lap--the reason for this was given in Chapter IV.--the starting position of the receiving drum will be similar to that given in the diagram Fig. 52, where X indicates the lap of the photographic film, and the arrow the direction of rotation.
It is, of course, obvious that a somewhat similar adjustment must be made with regard to the position of the stylus on the metal print at the transmitting machine.
In the present system, as in almost every photographic method of receiving that has been described, the Nernst lamp is invariably mentioned as the source of illumination. Since the advent of the high-voltage metal-filament lamps the Nernst lamp has fallen somewhat into disuse for commercial purposes, but it possesses certain characteristics that render it eminently suitable for the purpose under discussion.
The main principle of this type of lamp depends upon the discovery made by Professor Nernst in 1898, after whom the lamp is named, that filaments of certain earthy bodies when raised to a red heat became conductive sufficiently well to pa.s.s a current which raised it to a white heat, and furthermore that the glowing filament emitted a brighter light for a given amount of current than carbon filaments.
[Ill.u.s.tration: FIG. 52a.]
Nernst lamps are made in two sizes, the larger {99} being intended for the same work as usually done by arc lamps, and the smaller to replace incandescent lamps; the smaller type being made to fit into the ordinary bayonet lampholders. The princ.i.p.al parts of a Nernst lamp consist of the filament, the heater, the automatic cut-out, and the resistance, and their arrangement in the smaller type of lamp is given in the diagram, Fig. 52a.
The current enters at the positive terminal, pa.s.ses through the heater M, and out through the negative terminal. The filament B, which consists of a short length of an infusible earth made of the oxides of several rare minerals, of which zirconia is one, is a non-conductor at first, but becomes a conductor upon being raised to a high temperature by means of the heater M. As soon as the filament becomes conductive the current then pa.s.ses through the automatic cut-out H, and the armature D is attracted, thus breaking the heater circuit. The current then flows from the positive terminal {100} [Ill.u.s.tration] through the cut-out H, resistance J, and filament B, and from thence out of the lamp. Since the resistance of the filament decreases the hotter it gets, it is necessary to insert a ballasting resistance in series with it which has the opposite property of increasing its resistance as it gets hotter, to prevent the filament taking too much current and destroying itself. Such a resistance, J, consists of a filament of fine iron wire, which, to prevent oxidation from exposure to the air, is enclosed in a gla.s.s bulb filled with hydrogen gas. Fig. 52_b_ shows the form of ballast resistance used in the small and large type of lamp respectively.
Either direct or alternating current can be used with these lamps, and with direct current the polarity must be strictly observed, and that the positive wire is connected to the positive and the {101} negative wire to the negative terminal. With the smaller type of lamp once it has been correctly placed in its holder it is essential that it should not be turned, as a change in the direction of the current will rapidly destroy the filament.
[Ill.u.s.tration: FIG. 52c.]
The arrangement of the larger type of Nernst lamp can be readily seen from the drawing, Fig. 52c.
Care must be taken to see that the voltage required by the burner and resistance equals the voltage of the supply circuit, and that only parts of the same amperage are used together on the same lamp. No advantage is obtained by over-running a Nernst lamp, this only shortening its life without increasing the light. Under normal conditions the average life of the burner is about 700 hours.
The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75 watts per c.p. The light given is remarkably steady, and the lamps are adaptable for all voltages from 100 to 300. In one of the large type of lamps for use on a 235-volt {102} circuit the burner takes 0.5 ampere at 215 volts, and the resistance 0.5 ampere at 20 volts, while one of the smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts and 0.25 ampere at 20 volts for the burner and resistance respectively. The burner and heater are very fragile, and should never be handled except by the porcelain plate to which they are attached. The lamps burn in air and emit a brilliant white light of high actinic power, the intrinsic brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with 1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon filament lamps.
The chief advantage of the Nernst lamp from a photographic point of view lies in the fact that it produces abundantly the blue and violet rays which have the greatest chemical effect upon a photographic plate or film. These rays are known as chemical or actinic rays, and are only slightly produced in some types of incandescent electric lamps. Carbon-filament lamps are very poor in this respect.
Because a light is visually brilliant it must by no means be a.s.sumed that it is the best to use for purposes of photography, and this is a point over which many photographers stumble when using artificial light. Many sources of light, while excellent for illumination, have very low actinic powers, while others may have low illuminating but high {103} actinic powers. A lamp giving a light yellowish in colour has usually low actinic power, while all those lamps giving a soft white light are generally found to be highly actinic.
In addition to the actinic value of the source of illumination, the photographic film used must be very carefully chosen, as the chemical inertia of the sensitised film plays an important part in the successful reproduction of the picture, and also, to a certain extent, affects the speed of transmission. The length of exposure, the amount of light admitted to the film, and the characteristics of the film itself, are all factors which have a decided bearing upon the quality of the results obtained, and the film found to be most suitable in one case will perhaps give very unsatisfactory results in another.
In photo-telegraphy the length of exposure is determined by the time taken by the transmitting stylus to trace over a conducting strip on the metal print, and this time, of course, varies with the density of the image and also with the speed of transmission.
The film in ordinary photography is chosen with regard to the subject and the existing light conditions, and the amount of light admitted to the film and the length of exposure are regulated accordingly. No such lat.i.tude is, however, possible in photo-telegraphy. With each set of apparatus {104} the various factors, such as the light value, the amount of light admitted to the film, and the length of exposure, will be practically fixed quant.i.ties, and the film that will give the most satisfactory results under these fixed conditions can only be found by the rough-and-ready method of "trial and error."
The films in common use are manufactured in four qualities, namely, ordinary, studio, rapid, and extra rapid. These terms should really relate to the light sensitiveness of the film (or, as it is technically termed, the speed), but at the best they are a rough and very unsatisfactory guide, for the reason that some unscrupulous makers, purely for business purposes, do not hesitate to label their films and plates as slow, rapid, etc., without troubling to make any tests for correct cla.s.sification.
The speed of photographic films and plates is generally indicated by a number, and the system of standardisation adopted by the majority of makers in this country is that originated by Messrs. Hurter & Driffield, abbreviated H. & D. In their system the speed of the film and the exposure varies in geometrical proportion, a film marked H. & D. 50 requiring double the exposure of one marked H. & D. 100. The highest number always denotes the highest speed, and the exposure varies inversely with the speed.
Besides the Hurter & Driffield method of {105} 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. (Watkin"s power number) and Wynne F. No., both of which are used to a fair extent.
The "Actinograph" number or speed number of a plate in the H. & D. system is found by dividing 34 by a number known as the Inertia, the Inertia, which is a measure of the insensitiveness of the plate, being determined according to the directions laid down by Hurter & Driffield--that is, by using pyro-soda developer and the straight portion only of the density curve. If, for instance, the Inertia was found to be one-fifth, then the speed number would be 34 1/5 = 170, and the plate is H. & D. 170. The W.P. No. is found by dividing 50 by the Inertia. Thus 50 1/5 = 250, and the plate is W.P. 250, but for all practical purposes the W.P. No. can be taken as one and a half times H. & D. The Wynne F. numbers may be found by multiplying the square root of the Watkins number by 6.4. Thus
[sqrt]250 = 15.81, and 15.81 6.4 = W.F. 101.
For those photographers who are in the habit of using an actinometer giving the plate speeds in H. & D. numbers, the following table, taken from the _Photographer"s Daily Companion_, is given, {106} which shows at a glance the relative speed numbers for the various systems. The Watkins and Wynne numbers only hold good, however, when the inertia has been found by the H.
& D. method.
TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS
------------------------------------------------------ |H. & D.|W.P. No.|W.F. No.||H. & D.|W.P. No.|W.F. No.| --------+--------+-----------------+--------+--------- | 10 | 15 | 24 || 220 | 323 | 114 | | 20 | 30 | 28 || 240 | 352 | 120 | | 40 | 60 | 49 || 260 | 382 | 124 | | 80 | 120 | 69 || 280 | 412 | 129 | | 100 | 147 | 77 || 300 | 441 | 134 | | 120 | 176 | 84 || 320 | 470 | 138 | | 140 | 206 | 91 || 340 | 500 | 142 | | 160 | 235 | 103 || 380 | 558 | 150 | | 200 | 294 | 109 || 400 | 588 | 154 | ------------------------------------------------------
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. {107}
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 lat.i.tude 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 {108} 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.
{109}
APPENDIX A
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.
Selenium is prepared by heating it to a temperature of 120 C., keeping it there for some hours, and allowing it to cool slowly, when it a.s.sumes a crystalline form and changes from a bluish grey to a dull slate colour. A selenium cell in its simplest form consists merely of some prepared selenium placed between two or more metal electrodes, the selenium acting as a high resistance conductor between them. The form given by Bell and Tainter to the cells used in their experiments is given in Figs. 53 and 53a. It consists of a number of rectangular bra.s.s plates P, P", separated by very thin sheets of mica M, the mica sheets being slightly narrower than the bra.s.s plates, the whole being clamped together in the frame F by the two bolts B. {110} By means of a sand-bath the cell is raised to the desired temperature, and selenium is rubbed over the surface, which melts and fills the small s.p.a.ces between the bra.s.s plates. All the plates P are connected together to form one terminal, and the plates P" to form the other. By using very thin mica sheets, and a large number of elements, a very narrow transverse section of selenium, together with a large active surface, can be obtained.
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 s.p.a.ces between the turns being filled with prepared selenium. A thin gla.s.s cover is sometimes placed over the cell to protect the surface from injury.
[Ill.u.s.tration: FIG. 53.