Contact-
23rd November 1917
Beleiving that we are included amongst those whome the Chief Engineer regards as "misrepresenting a point of view in spite of repreated official statements" we fell that we cannot let these comments go by without a word in reply. Flogging a dead horse is certainly a waste of time, we agree thus far, but we never considered that the B.B.C. was dead when we did a little flogging to urge forward the endeavour to achieve Empire Broadcasting. Our point of view was that the horse was not a horse, but a rather obstinate mule which suffered from inertia, or whatever one may like to call it, and required a certain amount of judicious flogging before it would start, though we felt confident that once started it would proceed satisfactorily along its course.
Mr Stewart, in the first instance, has no regard apparently whatsoever for the number of perfectly reputable radio set manufacturers who have invested large sums of money in research and factory equipment, nor in the number of work-people that might conceivably be thrown out of employment were his suggestion adopted and these same manufacturers put out of business.
Up to a certain point the statement from the B.B.C. which you publish is reassuring, but it would be far more satisfactory and equitable if the wireless industry had this assurance in some more permanent form. Whilst the letter which you have published is entirely satisfactory as representing the fixed policy of the B.B.C. on this important subject, and whilst this policy would, I feel sure, be generally approved as right and proper by independent judges, it should not be overlooked that the B.B.C. itself is liable to changes in administration, that a new executive might hold entirely opposite views, and that there is at present nothing in the Articles by which the Corporation is bound, to prevent the adoption by such a new executive of the very policy which the Corporation at the moment so entirely condemns.
There is certainly no doubt that when the Articles of Association were drawn up and approved, the possibility of such an action on the part of the B.B.C. must have been entirely overlooked, as otherwise it seems unlikely the way would have been left open, as it the case at the moment. However, it is not too late to remedy this matter.
In spite of the criticisms which are often very unjustly levelled at the B.B.C., they have, in an incredibly short space of time, carried out a marvellous service to the public. Only those who have heard radio in many other countries can fairly judge. Let them stick to this magnificent work, and allow the manufacturer to stick to his particular problem, which is the design and production of first-class radio apparatus at the lowest possible price and, morover, which is equally important, backed up by service after the initial sale.
(signed) W H Lynas, Managing Director, for and on behalf of Graham Amplion Ltd. Sept 21st.
I had already built a short-wave set with which I received the reports of the Dempsey-Sharkey fight. You will see by the front page from the local paper the interest that was taken in my reception of the report of that fight.
[The journal "O Estado do Para" devotes half its front page to an account of this incident, and reproduces a photograph of a group taken at Mr. Foster Smith's residence during the reception of the fight.]
There are only about four of us in this town of some 200,000 inhabitants with receiving sets that give results. Had it been the report of a football match in England the interest would have been greater still, the Brazilians being very keen followers of the game.
Unfortunately most people here seem to think North America is the only country that knows much about wireless, so that it is impossible to buy anything but American parts, and I certainly think that North America owes a lot of its increasing influence in the South American countries to its wireless propaganda, especially on short waves.
Announcements are very often made in Spanish, and advertisements are sometimes made, but not in a way to interfere with the programme. I am quite sure that if England put up a really good abd powerful short-wave station our prestige would go up enormously in South America, and apart from giving a tremendous amount of pleasure to Englishmen in these out-of-the-way spots, it would, no doubt, open up a market for English apparatus. People at home have no idea of the thrill it gives one in a foreign country so far away to hear English songs and music come in over the wireless, even if it is spoken with an American accent or by gramophone records from Holland.
I certainly congratulate you in your efforts to get a short-wave station established, and I sincerely hope that it will not be long before your efforts meet with success, and a really good and powerful short-wave station is established in England.
H Foster Smith. A.M.I.E.E. Para, N Brazil, August 22nd 1927.
I want to say something about the preponderance of vocal over band and string music. It is very large, as a glance at the programmes for any day wil show.
I have suggested to the B.B.C. that an occaisional programme of band or orchestra without any vocal items at all would delight a large number of listeners.
Even if the studio did justice to the tenors, sopranos, and baritones - which most certainly it does not - it is, I am sure, an unwelcome interuption. If they were given in a separate part of the programme - part 1 or part 2 - it would then surely suit all tastes.
One day last week 5GB had a "Military Band Programme" in the afternoon. Fifteen of the items were vocal and, I think, six band music. The same proportion existed in the evening programme, and, indeed, will be found in most of the programmes from the various stations.
It may be that the majority of listeners like this, but from what I hear I do not think they do.
Then there is the "running commentary." In my humble opinion the "Royal Tournament", the "Trooping of the Colour" and the "Tidworth Tattoo" were completely ruined by the incessant chatter of the commentators. Why should they do more than just tell us what is coming and leave us to enjoy the event!
On some occaisions it seemed to me that the music and the noise of the crowd were deliberately reduced so that the "talk" of the commentators should not be interfered with. I noticed this at the broadcasting of the "Trooping of the Colour".
R.T.Watkin Williams, Honiton, September 19th, 1927.
While I personally admire what Mr. Baird has done in forwarding the art of television, nevertheless I think, in fairness to the facts, that Mr. C. Francis Jenkins is entitled, probably more than any other man, to credit as the earliest successful worker in this important research.
I am forwarding some clippings from papers of several years ago which testify as to these facts, which I know will be of interest to your magazine.
John Hays Hammond, Jun.
Gloucester, Mass, USA. September 7th 1927.
clipping:
WASHINGTON Sunday June 14th 1925:
First motion pictures transmitted by radio are shown in Capital
Government officials and scientists, summoned quickly by telephone, view successful experiment in Laboratory of C. Francis Jenkins. Small apparatus functions perfectly.
A group of distinguished government officials and scientists, called unexpectedly from their offices and laboratories, sat yesterday morning in the laboratory of C. Francis Jenkins, at 1519 Connecticut Avenue Northwest, and saw for the first time in history moving pictures of a moving object miles away , received over the radio and thrown upon a miniature screen.
Among the visitors who had been called hurriedly on the telephone by Mr Jenkins when he found the machine functioning perfectly , and who visited the laboratory at various hours in the morning, were Secretary of the Navy Wilbur, Dr D K Burgess, director of the bureau of standards, Stephen P Davies, Acting Secretary of Commerce, W. D. Terrill, of the radio department of the Department of Commerce, and two San Fransisco scientists who heard of the experiments and accompanied the officials to the laboratory.
Although the image broadcast was devoid of dramatic interest of itself, being a small model windmill with the blades in motion... [clipping has been cut here on republication, and continues... ]
...NOF, the old naval radio station which was turned over to Mr Jenkins for experimental purposes when the department erected a larger one. It was from NOF that Mr. Jenkins broadcast still photographs to Philadelphia, Boston and other cities in 1923.
To illustrate motion, a small model Dutch windmill was erected and the blades propelled slowly by wind from an electric fan. The image of this was through a lens onto a ground glass. From this ground glass the image was picked up by Mr Jenkins apparatus in much the same fashion that it is for a still photograph. That is, a small sensitive pencil travels across it making approximately fifteen lines to the inch, converting the light intensity into electrical intensity or electrical modulations.
These modulations were broadcast over a wavelength of 546 meters and picked up in Mr Jenkins Connecticut Avenue laboratory. Here the modulations were converted back into light values, and a pencil of light made to travel in the same fashion as the sending... [here the clipping ended].
[Jenkins had claimed to have transmitted a moving image as early as 1923. Baird's first public demonstration was 25th March 1925. Whoever did what first, these two experimenters were both working at almost the same time on mechanical television- a short lived cul de sac quickly replaced by electronic television]
A number of instrument-making firms showed resistance boxes, ammeters, voltmeters, etc., and the B.B.C., in a special room showed what could be done in the way of good quality reproduction from the local station.
The exhibit which attracted the most attention was that of the Baird Television Development Co., Ltd., to whom were allotted several rooms in a separate building. The programme announced that Television, Noctovision, and the Phonovisor would be demonstrated by Mr Baird, but this proved too amitious an undertaking. A very successful demonstration of noctovision between two rooms separated by an intermediate room was given to large numbers of the members , who passed through in batches hour after hour, and left Mr Baird and his staff little time for anything else.
A special demonstration of television and noctovision between London and Leeds was to have been given before twenty selected members of the Association on the evening of Monday, September 5th, but although Mr Baird himself went to London to superintend the transmitting end, successful transmission was not obtained, and the waiting scientists were dismissed with regrets.
We understand that the Phonovisor was not shown in operation, but was explained in principle by Mr Baird at a conversazione held at the University.
At the receiving end the audience is in darkness whether for television or noctovision, and sees the subject on a screen across which a light sweeps in synchronism with the sweep of the lenses at the transmitter; this light being obtained from a neon lamp is red, and follows in intensity the fluctuation of intensity of brightness of the subject.
The impression is somewhat similar to that of a very slow moving cinema picture. At Leeds, owing to the temporary character of the installation and the absence of concrete foundations for the motors, the speed was exceptionally low, and the flicker effect consequently enhanced.
We noticed a peculiar effect which we at first put down to a fading of the picture, but which we found was an optical illusion; if one allowed ones eyes to focus on the red light sweeping across the screen, and followed it across the screen, the picture vanished; it seemed necessary to ignore the sweeping light and look, as it were, beyond it to infinity; the picture then showed clearly.
In this way one could clearly detect when the person whose face was being transmitted opened and closed his mouth, or lit a cigarette, the smoke being clearly visible, in spite of the fact that the infra-red rays are supposed to penetrate fog.
Apart from the inconvenience of dazzling the 'patient' we do not know what are the relative merits of television and noctovision, but the latter is certainly the greater scientific novelty, and was far and away the most striking application of science to be seen at the Leeds meeting. It reflected great credit on those who were responsible for it.
We do not know, however, whether it represented conditions exactly as they would be between two distant stations. At Leeds the sending and receiving rooms were apparently separated by a very short distance, which would greatly simplify the problem of synchronisation, or do away with it entirely if a single motor could be used to drive both transmitter and receiver.
The apparatus which has been called a phonovisor differs from that described above in that the currents from the light sensitive cell are made to actuate, after amplification, the cutter of a phonograph recorder. By playing this record a sound is obtained which has been called the sound of a persons face, assuming a face to be the subject transmitted. Even the most beautiful face treated in this way sounds somewhat commonplace, and as a method of storing a moving picture it appears to be very far behind the film.
Although, to quote Mr Baird, 'if this record is played into a televisor, the original moving scene which caused the sound is reproduced on the screen of the televisor'. We doubt whether such a cycle, even with the possibility of having the scene and the voices recorded on the same cylinder or disc, can be improved and simplified sufficiently to be more than a scientific novelty. However, scientific prophesy is a dangerous pasttime, and we will close by congratulating Mr. Baird on providing what was undoubtedly the most popular of the scientific exhibits at the Leeds meeting.
[to see a Baird disc and view a Baird moving image, visit the Bradford Museum of Photography etc. Or try a web site which has the images-tvdawn- Baird TV]
The experimental transmitter is erected in the research laboratories which have many historical associations, as it was under this same roof that the first long-wave broadcast transmitter was installed and tested before the Daventry site was chosen. Here also 5GB had its genesis.
The present occupier of this historical room transmits on a wavelength of 24 meters, and has the appropriate call sign of 5SW. Power is obtained from a three phase A.C. supply, which is generated on the premises, rectified and smoothed, after which it is passed to the main amplifier at a pressure of 8,000 volts.
The equipment is an interesting mixture of standard apparatus and experimental gear, but everything is laid out with a view to efficiency.
The experimental transmitter consists of two panels of a Marconi beam transmitter, with the addition of three modulating panels, and, of course, the necessary rectifying valves for the various anode supplies. The main amplifier is fitted with two special oil-cooled valves.
The second beam transmitter panel is fitted with two amplifying valves in the top section, and below this the drive or master oscillator totally enclosed in a copper screening box. Each power amplifier consists of two 10 kilowatt valves. It is necessary accurately to balance the supply to both valves, so that meters are included in the filament and anode circuits of each valve to facilitate this adjustment. These instruments and the various tuning controls are mounted on insulating panels on the front of each unit of the beam transmitter. The three modulating panels are mounted in a temporary wooden framework, connected to the oscillators by a speech transforme.
The modulating panels are each fitted with two 7.5 killowatt cooled anode valves, these being connected in parallel and forming the main modulator. This is preceded by a smaller panel carrying two air cooled valves forming the sub-modulator, the function of which is to amplify the signal current received from the land line, or local microphone circuit, before passing them to the main modulator.
The Aerial System
The output from the main amplifier is fed to the aerial by means of a current feeder encased
in a copper tube connected to earth. The purpose of this is to prevent interference from
external sources. At the transmitter end the feeder terminates in a coupling coil and
balancing circuits, and the far end is connected to the base of the aerial. Ammeters are
arranged at either end of the feeder, and the circuits are adjusted so that the current is
the same value at both the input and output ends of the feeder. Slight variation in the
wavelength of the aerial will have no effect on the closed oscillatory circuit, as the
feeder can be considered a resistance.
The radiating system used is a Franklin aerial, and this incorporates a number of unique features based on the experience gained from experiments with beam transmitters.
The Chelmsford aerial takes the form of five half wave aerials, in series with non-radiating portions, connected between each radiating section. The whole system is suspended from a wire, but insulated therefrom, attached to the tops of two 450 foot ( 137 meters ) masts.
It is claimed that by the use of this arrangement every foot of vertical wire radiates energy, and the radiation resistance of the system is of a very high order.
This completes the main equipment, and the writer was then conducted to a small hut which was used as the studio during the recent 36 hour test undertaken with a view to ascertaining what degree of reliability could be expected from a short wave broadcast service.
This room contained a miniature switchboard with direct lines to the local telephone exchange, a Reisz microphone and amplifier and a special amplifier for use with a gramaphone pick up device; this provided the matter for broadcasting during the tests.
Reports have been received from many parts of the world, but it seems that the most consistent reception was experienced in Canada, although on those occaisions when the Antipodes received 5SW the results left nothing to be desired. Good reception has also been reported by short wave enthusiasts in all parts of Great Britain.
The object of these tests was to determine, by means of measurements and reports from trained observers and ordinary listeners situated in all parts of the United States, just exactly what the effect of this enormous increase in power would be. During the week commencing August 14th comparison tests were made between the new 100 kW transmitter and the 50 kW transmitter, which latter is now run at 30 kW., in accordance with the ruling of the Federal Radio Commission.
These investigations, which included measurements of signal strength, audibility and modulation, are part of an extensive development programme, as the result of which the General Electric Company's engineers hope to improve the broadcast service.
By courtesy of the General Electric Company we are able to give the following description of the new transmitter, together with the accompanying photographs, and a brief resume of some of the earliest reports of the results of the tests.
The South Schenectady transmitter laboratory covers 54 acres, and facilities are available for suitable aerial and counterpoise systems, and for the power and cooling requirements of a large number of transmitters, or for a single very large transmitter. There are, for example, four steel aerial towers, three 300 ft. high and one 150 ft. high, in addition to a large number of smaller masts. There is also a rectifier capable of supplying 750 kilowatts of direct current power at 15,000 volts.
High-power Valves Save Space
The development of the 100 kW transmitter has been hastened, to some extent, by the
production by the General Electric Company of a 100 kW transmitting valve.
The new transmitter occupies less than half the space taken up by the 50 kW. transmitter, heretofore one of the highest powered broadcast transmitters in existence. Two 100 kW. valves are used in the high power amplifier unit, and three more in the modulator unit. The 50 kW. transmitter (now operated at 30 kW. as already mentioned) uses seven 20 kW. valves in the amplifier and twelve valves of the same size in the modulator.
The new 100kW valves are of conventional metal anode construction. The anode itself is of copper, approximately three feet long by 3.25 inches in diameter. The grid and filament leads are brought out through a glass cylinder at the top, the glass part being approximately 19 inches long by five inches in diameter. The overall length of the valve is 50 inches, and the filament requires 210 amps at 33 volts. During normal operation two such valves are used in parallel in the power amplifier, which is connected to a closed, or "tank" circuit, which is inductively coupled to the aerial by means of coupling coils and a high frequency transmission line.
The aerial is of the vertical type, consisting of a cage 2 feet in diameter and 240 feet high. The wires of the cage are combined to form a single conductor for the lower part of the aerial, and a counterpoise, consisting of a radial wire system 240 feet in diameter, is used instead of a direct earth connection.
Crystal Control:
The frequency of the transmitter is controlled by a quartz crystal, a method which is rapidly becoming standard practice with American broadcasting stations of the highest class. The output of the crystal-controlled oscillator is amplified by five stages of radio-frequency amplification to a power which is sufficient to energise completely the grids of the two 100kW power valves. All these stages of amplification are completely neutralised, so that there is little possibility of independent oscillation occurring in the amplifier chain. The last stage employs a 20kW water-cooled valve, the output of which goes to the power amplifier.
The frequency of the transmitter is the same as that used by the ordinary WGY transmitter, 790 kC.
Speech or music to be broadcast is sent from the WGY studio over the telephone cable, at a level approximately equal to that used for ordinary telephone conversation. This input to the station is then amplified 1,000 times by an ordinary L.F. (low frequency) speech amplifier, the last stage of which also employs a 20kW water-cooled valve. The output of this intermediate LF power amplifier is then impressed upon the grids of the three 100 kW valves comprising the modulator. These three valves operate directly in the plate circuit of the radio-frequency power amplifier valves, and vary the plate potential in accordance with the speech frequency.
POWER RECTIFIER:
The high-tension power for the plates of these enormous valves is obtained from a rectifier which employs six two-electrode valves.
These valves are the sime size as the 100 kW three-electrode valves, only they have no grid structure. The rectifier is capable of supplying 750 kW of direct current power at
a potential of 15,000 volts. Several large filter units smooth out the 60 cycle AC ripple before the output of the rectifier is applied to the plates of the transmitting valves.
A motor-operated voltage regulator enables the operator to vary the output voltage at will, under load. This rectifier is probably the largest of its type in use by a broadcasting station. It is capable of supplying a broadcast transmitter having an output of 250 kW. Although a transmitter of such power is not available at present, it is now considered as being practical.
Cooling Arrangements
In order properly to cool the anodes of the large power valves, it is necessary to circulate twelve gallons of water per minute through the water jacket in which each valve is
mounted. Thus, for the transmitter proper, exclusive of the rectifier, a flow of sixty gallons of water per minute is required.
This is obtained from a centrifugal pump which draws its supply from a cistern of approximately 20,000 gallons capacity. On its return from the valves this water is caused to flow through a radiator unit, which is kept cool by a current of air supplied by a large blower. The water is then returned to the cistern.
This type of cooling system is called a "closed system", since it is not necessary for the water to come into actual direct contact with the air in order to be cooled. In this way the water is protected from dust and other impurities which might adhere to the plates of the valves. Water cooled in this way can be used over and over again for long periods without replenishment.
Simple Remote Control
The operation of this new high-power transmitter is rendered quite simple through the use of remotely controlled electrical relays and automatic protective devices.
The operator has before him two major controls. One switch controls a small rectifier feeding the plate circuit of the (comparatively) low-power valve which supplies the grid
excitation for the main power amplifier; while a second switch controls the high-power rectifier which supplies the plate circuit of the main amplifier and modulator power valves.
In getting the set ready to go on the air as the Americans put it, all motor generator equipment (including pumps and blowers) is started, and rectifiers and amplifiers, both speech and radio, are switched on. When all is ready, the carrier wave is finally switched on to the aerial by the manipulation by the operator of the two major control switches mentioned above.
Protective devices are employed automatically to trip off the power supply in case of valve failure, also to give warning to the operator in case of failure of the water supply. During the transmission of a programme, the operator is continually checking the degree of modulation by means of an oscillograph, and the quality of the transmission is also still further checked by means of a suitable monitoring loud-speaker.
Results of Tests
At the time of writing it is too early to give full details as to the results of the thirty day test of this new high power transmitter, but a sufficient number of reports have already been sent to the General Electric Company to enable them to ocme to come conclusions.
It must be remembered, when reading what follows, however, that all the tests were conducted between midnight and 1 am at a time when the majority of the Eastern broadcasting stations are closed down, and conditions for long distance reception are almost at their best.
Especially interesting to the engineers engaged upon the tests is the fact that the temporary increase in power has brought an improvement in quality, volume and sharpness of tuning. A survey of the letters received at the conclusion of the third early morning test indicates that:-
Signal strength over the region East of the Mississippi River and North of North Carolina (ie within a range of 500 to 600 miles) is equal to that of ordinary broadcasting stations working within fifty miles of the receiver.
WGY was heard with good volume and clarity in parts of the country not reached since early in the Spring, the volume being so great, in some cases, as to override static and even severe elctrical storms.
Fading is not appreciably improved by high power in areas within 300 miles of Schenectady, where WGY's transmissions normally faded; but many of the more distant listeners reported that fading was less frequent and less pronounced.
Many listeners in areas outside the large population zones reported that the 100 kW transmissions were the first static clear music they had received for months.
Static Overridden
It so happened that the first test transmission was conducted under the most severe conditions to which radio broadcast transmission could be subjected, for the greater part of the area reached was covered by electrical storms.
One listener living in Portsmouth, Va (about 300 miles from WGY) reported that lightning was so severe that the street lights in parts of the town were out of commission, yet the storm had no effect upon the music, which came in free from atmospherics.
Another listener about 600 miles away, in Minnesota, reported that WGY's volume exceeded that of his local station, eighty miles away; while Mr. C. C. Hollenback, chairman of the radio committee of station WAIU, Columbus, Ohio (about 500 miles distant) wrote: I wish to advise you that the power overrode the static in a very pleasing manner, and I also want to say that your station had a signal strength equivalent to the strength of our own station WAIU, which uses 5,000 watts, and is fifteen miles distant from my farm home.
A listener fifteen miles from KDKA reported that he got more volume on less (receiver) power from WGY than he did from KDKA (25 kW).
A prominent engineer of the radio division of the U.S. Department of Commerce summarised a technical report on reception as follows: It is my opinion that the efficiency of your station, so far as the delivery of reliable signals to broadcast listeners is concerned, has been increased 100 per cent. This not only holds for coverage but for quality as well.
But one correspondent pronounced the test a failure. He is a resident of Newburyport, Mass, about 125 miles from WGY. He found that WGY faded badly on high power. There never was a high powered station but what was a failure, he stated. You cannot expect a balloon to keep from bursting when you give it too much gas. What becomes of a wave if blown apart.
These preliminary results are of particular interest to British listeners at present, in view of the discussion anent the proposed regional scheme.
When all reports, technical and otherwise, have come in and been duly tabulated and collated, the engineers technical report on the experiments should prove highly interesting. Although no tests have so far been made in daytime, the night-time tests should yield valuable data on the relationship between fading and high power at varying distances.
In conclusion, it might be added that WGY's situation, some distance inland, and in a valley, does not seem to favour the radiation eastwards of his 790kC wave. The station is not easy to receive even in New York city. If, therefore, any readers in this country happened to hear the new transmitter testing, their reports will no doubt be welcomed by the engineer in charge of the station.
[Update from site owner- we now realise that fading can be due to variations in the reflecting layer of the atmosphere caused by variations in solar etc radiation, which are subject to varying cycles. Medium Wave radio travels better over longer distances where the signal passes over a route in nighttime, which in the Northern hemisphere is larger in Winter. Yours truly heard a Boston Mass radio station on Medium Wave one Winter night, in Cheshire. This was before European stations cranked up the power and transmitted 24 hours a day... Short Wave broadcasts generally travel further, more reliably, up to a cut off frequency which varies with solar radiation, and rarely exceeds 32MHz.].
It needs a keen sense of relative values to appreciate the merits of design of a speech amplifier and series of loud-speakers which can magnify many million times the minutest sound impulses represented by a tiny flickering beam of light not more than one thousandth of an inch broad, and deliver them to an audience of, say, 1,500 people with a volume and faithfulness which is practically the same as the original. The synchronisation of realistic speech and music with the film undoubtedly adds a new interest for the cinema goer and it would appear safe to predict that in a few years time most of the larger cinemas will be equipped with apparatus for synchronous sound reproduction.
The Movietone system of talking films involves the photographic recording of sound on a one tenth of an inch margin of the standard 1.375 inch film, using a standard camera and a standard projector, to which are easily fitted special additional apparatus; simultaneous with the recording of sound the associated scene is photographed on the rest of the film, and when developed and printed as a positive presents an appearance as shown above.
There are a number of methods whereby the marginal record of sound is obtained.
The small diaphragm which causes the light to oscillate naturally has mass, and therefore a natural period of vibration, which may fall within the frequencies associated with music and speech, thus giving undue prominence and an unnatural effect to certain notes; reference will be made later to means of neutralising this defect.
As an alternative to the above electromagnetic method of recording, it should be mentioned that in some films the sound-track is made by the A.E.O. flashing lamp scheme.
The sound represented by tiny lines of varying depth and opacity on the film have now to be converted in turn to light fluctuations, electric pulsations, and ordinary sound waves, and it is this part of the article which may be of special interest to the wireless enthusiast, as it is concerned in discussing the best means of getting a tremendous volume of undistorted signals using no less than seven stages of Low Frequency (L.F.) amplification.
The first process of creating light fluctuations is achieved within a standard projector by arranging that a small pilot light, with a short, stout filament giving a brilliant point source of light, is set so that its rays are directed by means of lenses through a one thousandth of an inch slit on to the margin of the moving film. The light beam of varying intensity thus formed is now directed on to a light sensitive cell which changes its electric resistance sympathetically.
There are two types of these cells available, namely, selenium and photo-electric. The former has the advantage of developing a higher light-dark current ration, but suffers from lag, while the latter develops a relatively much smaller current change but is instantaneous in action, and is used in the system under discussion.
In order to understand the functioning of the photo-electric cell, light must be considered as an electromagnetic disturbance, and matter as consisting of atoms built up of aggregates of electric charges woth positive nuclei surrounded by negatively charged electrons.. When an electromagnetic wave falls on an atom of metal, for instance, it is not unreasonable to expect that the equilibrium of opposite charges will be upset, and that electrons will become detached. Actaully in practice this is the case, and the effect is found to be most pronounced with electropositive metals such as sodium, potassium and rubidium.
The theory has been put forward that the vision of the human eye is due to the detecting of electrons from the rod and cone cells by photo-electric action.
The Movietone cell is a glass body which internallydeposited metal in vacuo; the detached electrons are attracted to the central collecting ring, which is maintained at a constant positive potential. So small are the oscillations from the collecting ring that is hat been thought advisable to have as short a lead as possible to the first three stage L.F. amplifier, necessitating the housing of this unit within the body of the projector, which is subject to heavy vibration and to the influence of commutator disturbance and an arc lamp passing 100 amperes. Complete spring suspension and earthed metal screening, however, guard against any spurious effects.