CHAPTER V
THE "TELEPHOGRAPH"
In the present chapter it is proposed to give a brief description of a system of radio-photography devised by the author, and which includes a greatly improved method of transmitting and receiving, as well as an ingenious arrangement for synchronising the two stations; the whole being an attempt to produce a system that would be capable of working commercially over fairly long distances.
The system about to be described, and which I have designated the "telephograph," is the outcome of several years' original experimental work, many difficulties that were manifest in the working of the earlier systems having been overcome by apparatus that has been expressly designed for the purpose.
In any practical system of radio-photography the following points are of great importance: (1) the speed of transmission; (2) the quality of the received picture; (3) the method of synchronising
the two machines so that transmission and reception begin simultaneously; (4) the correct regulation of the speed of the driving motors; (5) the simplicity and reliability of the entire arrangement. Points 1 and 2 are dependent upon several factors; the number of contacts made by the stylus per minute; the size of the metal print used; the number of lines per inch on the screen used in preparing the print; and the accurate and harmonious working of the various pieces of apparatus employed.
In the system under discussion the size of the metal print used is 5 inches by 7 inches, and a screen having 50 lines to the inch is used for preparing it. With the drum of the machine making one revolution in four seconds, the stylus makes 87 contacts per second, or 5220 a minute, the time for complete transmission being twenty-five minutes. By the use of ordinary relays not more than 2000 contacts a minute can be obtained, and in the present system it is only by means of a specially designed relay that such a high rate of working has been made possible. Similarly, too, with the receiving of such a large number of signals transmitted at such a high speed, a special instrument has been devised that can record this number of signals without any trouble, and could even record up to 8000 signals a minute, provided that a suitable transmitter could be designed.
In the present system the writer does not claim to have completely solved the problem of the wireless transmission of photographs, but it is a great advance on any system previously described, and the following advantages are put forward for recognition: (1) a greatly improved method of transmitting and receiving; (2) a simple method of regulating the speed of the driving motors and maintaining isochronism with a limit of error of less than 1 in 800; (3) an arrangement for synchronising the two machines whereby transmitting and receiving begin simultaneously; (4) the use of one machine only at each station.
Transmitting Apparatus
A diagrammatic representation of the apparatus required for a complete station, transmitting and receiving combined, is given in Fig. 35, the usual wireless equipment having been omitted from the diagram to avoid confusion.
The Machine.—This, as will be seen from Fig. 36, consists of a base-plate M, to which are attached the two bearings B and B'. The bearing B' is fitted with an internal thread to correspond with the threaded portion of the shaft D. The drum V is a brass casting, being fastened to the shaft by set screws. The shaft is threaded 75 to the inch. The bearings are preferably of the concentric type. The circuit breaker C is so arranged that when
the drum has traversed the required distance, the end of the shaft pushes back the spring M, breaking the circuit of the driving gear and stopping the machine. The machine is connected to the driving gear by the flexible coupling A.
M, motor; Y, isochroniser; F, clutch; A, machine; R, stylus; S, relay; X, gearing; O, circuit breaker; T, receiver; C, condenser; U, telephone relay; K, polarised relay; L, contact breaker; D, D1, D2, D3, batteries; P, friction brake; B, B1, double-pole two-way switches; N, N1, N2, single switches; W, key; E, electric clock; J, telephones.
The drum measures 5 inches long by 21/8 inches diameter, and this takes a metal print 5 inches by 7 inches, which allows for a lap of about 1/4 inch. In working, the print is wrapped tightly round the drum, being secured by means of a little seccotine smeared along one edge. Care must be taken that the edge of the lap draws away from the point of
the stylus and not towards it. A margin of bare foil, about 1/8 inch wide, should be left on the print at the commencing edge, the purpose of which will be explained later.
The Stylus.—As the drum of the machine travels laterally, by reason of the threaded shaft and bearing, the stylus must necessarily be a fixture. It consists of a holder B, drilled to take a hardened steel point S, attached to the spring M. The spring is arranged to work in the guide F, which is provided with an adjusting screw W for regulating the pressure of the stylus upon the print; the pressure being sufficient to enable good contact to be made, but must not be heavy enough to scratch the soft foil. The needle should present an angle of about 60° to the surface of the print, as this angle has been found to give the best results in working.
To eliminate any sparking that may take place at the point of make and break, due to the self-induction of the relay coils, a condenser C, about 1 microfarad capacity, should be connected across
the drum and stylus. The complete stylus is given in the drawings, Figs. 37, 37a, and also in the diagrams Figs. 8 and 9.
Showing the arrangement for sliding the stylus to or from the machine.
The Relay.—As will be seen from the diagram, Fig. 38, this consists of two electro-magnets having very soft iron cores, the magnet M being wound in the usual manner, while the magnet N is wound differentially. The armature A is made as light as possible, and is pivoted at P, and when there is no current flowing through any of the coils, is held midway between the magnet cores by the two spiral springs S and T, which are under slight but equal tension. The connections are as follows. The wires from the winding on M are connected directly to the relay terminals F and H, as are also the wires from one winding on N. The other winding on N is connected in series with the battery C, ammeter B, and regulating resistance R.
When the circuit of the battery C is completed, the coil of N, to which it is connected, is energised, and the armature A is attracted against the stop V. When in this position the tension of the spring S is released, while the tension of the spring T is increased. As soon as the circuit of the battery D is completed by means of the metal line print on the transmitting machine, the current divides at the terminals F and H, a portion flowing through the magnet coil M, and a portion through the remaining winding on N. The current which flows through the winding on N produces a magnetising effect equal to that caused by the other winding on N, but since the two windings are of equal length and resistance, and since the current flowing through the two windings is of equal strength but in opposite directions, the result is to neutralise
the magnetising effects produced by each winding, and consequently no magnetism is produced in the cores.
The other portion of the current from D flows through the coil M, and it becomes magnetised at the same time that the coil N becomes demagnetised. The armature A is attracted by M against the stop X, and this attraction is assisted by the spring T, which was under increased tension. The conditions of the springs are now reversed, the spring S being under increased tension, while the tension of the spring T is released.
As soon as the current from D is broken, the magnetism disappears from M, the neutralising current in N ceases, and N once more becomes magnetised, owing to the current which still flows through one winding from C; the armature is therefore again attracted by N, assisted by the spring S. The current flowing through the two windings of N must be perfectly equal, and the regulating resistance R, and ammeters B and B', are inserted for purposes of adjustment. The current from C must flow in a direction opposite to that which flows from D.
H, H', containers; M, mercury; E, paraffin oil; T, T', terminals; C, suspending rod; D, base; F, F', dipping rods.
The local circuit of the relay is completed by means of a copper dipper in mercury, somewhat resembling an ordinary mercury break, but modified to suit the present requirements. The arrangement will be seen from Fig. 39. The whole of the
moving parts are made as light as possible, and for this reason the rod C and the dippers F, F' should be made as short as convenient. The containers H, H' are separate, of cast iron, and rectangular in shape. The dipper is of very thin copper tube—an advantage where alternating current is to be used—and is made adjustable for height on the suspending rod C. The leg F is of such a length that permanent contact is made with the mercury in the container H, while the leg F' clears the surface of the mercury by about 1/4 inch, when the armature of the relay is in its normal position. To prevent undue churning of the mercury, which would necessarily take place if the dipper entered and left the mercury at each movement of the armature, a pointed ebonite plug is inserted in the end of the tube. This will be found to give good results at a high speed, the mercury being practically undisturbed, and the production of "sludge" reduced to a minimum. To prevent oxidation of the mercury, and to prevent arcing, the surface is covered with paraffin oil. If this is not sufficient to prevent arcing a condenser should be shunted across the
containers. The volume of mercury, and the area of the dippers, should be sufficient to carry the current used for a considerable period without heating up to any extent. An adjustable weight J is provided in order to balance the armature and dipping rod.
The remaining transmitting apparatus consists of the battery D2 and the usual wireless apparatus. The double-pole two-way switch B' is to enable the photo-telegraphic set to be switched out and the hand key W switched in for ordinary signalling purposes. The battery D2 should be about 12 volts.
Receiving Apparatus
The wireless portion of the receiver is similar to that given in Fig. 22, is of the usual syntonic type, and comprises an oscillation transformer, S being the secondary, and P the primary; C' is a block condenser, and C a variable condenser. The detector D is of the carborundum crystal or electrolytic pattern. A two-way switch B is provided so that the relay U can be switched out and the telephones J switched in for ordinary receiving purposes. The relay U is a Brown's telephone relay.
The Receiver.—The magnified current from the relay U is taken to a special telephone receiver, the construction of which is given in Fig. 40. The diaphragm F is about 21/2 inches diameter, and should be fairly thin but very resilient. Only one
The optical arrangements are as follows. By means of the Nernst lamp L, and the lenses B and B', Figs. 42 and 43, a magnified shadow of G is thrown upon the screen J. When the shutter G is in its normal position (i.e. at rest), its shadow is just above the small hole in J, and light from L reaches the photographic film wrapped round the drum V of the machine.
J, screen; L, Nernst lamp; G, shutter; B, condensing lens; B1, focussing lens.
When, however, signals are sent out from the transmitting apparatus, the magnified current from the relay U energises the coil of the special telephone S, attracting the diaphragm F, and consequently giving movement to the pivoted rod R. As by means of the optical arrangements a
magnified movement as well as a magnified image of G is thrown upon the screen J, the shadow of G will, when the telephone S is actuated, cover the hole in the screen, and prevent any light from reaching the film on V, until current from the relay U ceases to flow. Therefore, when the stylus of the transmitter traces over a conducting strip on the metal print, no light reaches the film on V, but when tracing over an insulating strip the shadow of G on the screen J rises, and the light from L reaches the film. By this means a positive picture is received, which is a great advantage where the photographs are required for reproduction. Atmospherics would be represented by irregular transparent marks on the film after development, and these can be easily eradicated by retouching.
The drum of the machine moves laterally 1/75th of an inch per revolution, and the hole in the screen is 1/90th of an inch in diameter. As the screen J is not in direct contact with the film, the slight diffusion of the light that takes place will produce
a mark of about the right thickness. With a movement of the diaphragm of only 1/40000th of an inch, the actual movement of G will be 1/4000th of an inch. If the optical arrangements have a magnifying power of 100, then the movement of the shadow upon the screen will be 1/40th of an inch, which will be ample to cover the aperture.
The aluminium rod R, minus the counter-weight, can be made to weigh not more than 12 grains. It is necessary to enclose the optical parts in a light tight box, indicated by the dotted lines in Fig. 43, in order to prevent any extraneous light from reaching the film.
The Contact Breaker.—The contact breaker (L, Fig. 35), as will be seen from Fig. 44, consists of an electro-magnet N, the windings of which are connected with the battery B and the polarised relay K. The armature which is supported by the spring G carries a contact arm A, which in its normal position makes permanent contact with the contact screw T, and completes the circuit between the relay K and the telephone relay U (Fig. 35). As soon as the transmitter sends out the first signal, the magnified current from the telephone relay actuates the relay K, which in turn completes the circuit of the contact breaker. Directly the armature M has been attracted, the contact with T is broken, and A makes fresh contact with the screw H, by means of the spring Z
fastened to the underside of A. The armature, once it has been attracted, is held in permanent contact with H by the catch S, independent of the magnets N. As soon as contact is made with H, the clutch (F, Fig. 35) circuit is completed, and the circuit of the relay K is broken. When the circuit of the clutch F is broken by means of the circuit breaker C on the machine (Fig. 36), the stop S is pulled back by hand, allowing the contact arm A to rise, and again make fresh contact with the contact screw T.
Driving Apparatus
The Friction Brake.—This consists of a steel disc A, Fig. 45, about 21/2 inches diameter and 3/8 inch or 1/2 inch wide on the face, secured to the main shaft of the driving motor. The arm H, pivoted at C, carries at one end the curved block B, which is faced with a pad of tow F. The other extremity is pivoted to the steel rod P, which slides
The Clutch.—The details of this will be seen from Figs. 46 and 47. It consists of a steel shaft coned at both ends running between two countersunk bearings, one of which is adjustable. This shaft carries the two portions of the clutch A and B, the portion A being a fixture on the shaft, and the portion B running free upon it. The portion B is a gun-metal casting bored to run accurately upon the steel shaft. A soft iron annular ring is fastened to the face.
E, spindle; R, bobbins; P, iron cores; D, copper rings; T, brushes; N, back plate; V, front plate; J, gearing; S, spring; H, collar; Z, iron ring; F, fixed bearing; C, insulating bush.
The portion A consists of a gun-metal casting
The Isochroniser.—This is a device for ensuring the correct speed regulation of the driving motors, and is shown in detail in Fig. 48. It comprises two portions, one portion being rotated at a definite speed by electrical means, and the other portion rotated by the driving motor.
The main portion consists of a metal tube N, bushed at both ends, the bottom end of the tube being arranged to work on ball-bearings. An ebonite bush C carries three copper rings T, T1, T2, and the brushes R, R1, R2 are in electrical contact with them. The ebonite plate J, 31/2 inches diameter, is secured to the top end of N, and carries a contact piece Q, shown separate at E. As will be seen this is a block of ebonite with three contacts arranged on the top surface. The middle contact P is 1/64th of an inch wide, and the contacts P1
and P2 are placed on either side at a distance of 1/16 inch; the contact strips P1, P2 carry the brass pins D, which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting wire is carried from the contact P to the copper ring T, another from P1 to T1, and one from P2 to T2.
N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T1, T2, copper rings; C, insulating block; R, R1, R2, brushes; J, ebonite disc; Q, contact block; D, metal pins; O, pulley, P, P1, P2, contact plates; K, needle; Z, spring; W, steel rod; E, countersunk bearing.
The bushes S are bored a running fit for the steel rod W (shown separate at A), which is coned at both ends, and runs between two countersunk bearings, the bottom bearing E being fixed while
the top bearing (not shown) is adjustable. A needle K is fastened near the end of the rod W, and attached to this needle is the spring Z, which presses lightly but firmly upon the contact block Q. To provide a level surface for Z to work over, the spaces between the contact pieces are filled in with an insulating material, and the whole surface finished off perfectly smooth. The spring Z is 1/8 inch wide for portion of its length, but at the point where it presses upon Q it is reduced in width to 1/64th of an inch (see Fig. 48). The driving arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a grooved pulley, is run in bearings parallel with the shaft W, and is connected by suitable gearing to the shaft of the driving motor, so that the needle K makes one revolution in about 21/2 seconds. A belt passing over the pulleys connects the two shafts, and the tension of the belt is regulated by means of an adjustable jockey pulley.
The tube N, carrying the disc J, must be rotated at a fixed speed, and this is accomplished in the following manner. An ordinary electric clock impulse dial, actuated from a master clock, is connected by suitable gearing H, so that the tube N makes exactly one revolution in 2 seconds; it being possible to adjust an electric clock of the "Synchronome" type, so that it only gains or loses about 1 second in 24 hours, and this provides
an accuracy sufficient for all practical purposes. The connections are given in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that a connecting wire is run from the steel spindle W to one terminal each of the lamps L, L1, L2, and from the other terminal of the lamps to one terminal of the batteries J, the battery comprising a set of three 4-volt accumulators. The other terminals of the batteries are joined one to each of the brushes R, R1, R2.
M, terminals for connecting to electric clock; L, white lamp; L1, blue lamp; L2, red lamp.
The lamps are coloured, the lamp L being white, and the lamps L1 and L2 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
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 P2, the lamp L is extinguished and the red lamp L2 lights up, and remains alight until the operator reduces the speed. Similarly, too, if the needle travels slower than J, contact is made with P1, and the circuit of the blue lamp L1 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
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.
M, clockwork motor; S, isochroniser; E, friction break; T, brushes; F, electric clutch; X, gearing; D, D1, switches; A, flexible coupling; K, polarised relay; L, circuit breaker; B1, B2, B3, 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 D1 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 passed under the stylus, the end of the shaft D, Fig. 36, engages
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 D1 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.
The current from the relay U, due to the transmitting stylus passing 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.
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 pass 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.
Nernst lamps are made in two sizes, the larger
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 principal 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, passes 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 passes 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
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
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.
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
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 assumed 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
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 latitude is, however, possible in photo-telegraphy. With each set of apparatus
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 quantities, 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 classification.
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
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
√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,
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.
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.