Homepage
Login
Profile
BookClubs
dmBox
Read this ebook for free! No credit card needed, absolutely nothing to pay.
Words: 22970 in 8 pages
This is an ebook sharing website. You can read the uploaded ebooks for free here. No credit cards needed, nothing to pay. If you want to own a digital copy of the ebook, or want to read offline with your favorite ebook-reader, then you can choose to buy and download the ebook.
apparatus required is very cumbersome and the work of winding both tedious and heavy. This method of driving was at one time universally employed with the Hughes printing telegraph, but it has now been discarded in favour of electro-motors, which are more compact, besides being cheaper to instal in the first instance.
Synchronising and isochronising the two machines are the most difficult problems that require solving in connection with wireless photography, and as previously mentioned, the synchronising of the two stations must be very nearly perfect in order to obtain intelligible results. The limit of error in synchronising must be about 1 in 500 in order to obtain results suitable for publication.
The electrolytic system is perhaps the easiest to isochronise, as the received picture is visible. On the metal print used for transmitting, and at the commencing edge a datum line is drawn across in insulating ink. The reproduction of this line is carefully observed by the operator in charge of the receiving instrument, and the speed of the motor is regulated until this line lies close against a line drawn across the electrolytic paper. Although this may seem an ideal method there are one or two considerations to be taken into account. Unless the decomposition marks are made the correct length and are properly spaced, however good the isochronising may be, the result will be a blurred image. Any one who has worked with a selenium cell, will know that it cannot change from its state of high resistance to that of low resistance with infinite rapidity, and the effects of this inertia, or "fatigue" as it has been called, are more pronounced when working at a high speed. In working, the effects of this inertia would be to increase the time of contact of the relay F as the current from D would flow for a slightly longer period through R to F than the period of illumination allowed by K. This, of course, would mean a lengthening of the marks on the paper; results would also differ greatly with different selenium cells. There is a method of compensation by which the inertia of a cell can almost entirely be overcome, but it would add greatly to the complicacy of the receiving apparatus.
metal frame which supports an upright steel bar S, whose ends turn on pivots. This bar is rectangular in section. The gear-wheel G is fastened near the bottom of this rod and gears with a similar wheel on the shaft of the driving motor . Suspended from the broader sides of S are the two flexible arms D, each carrying a brass ball T. These balls are not fastened to the arms, but can slide up and down, being held in position by the wire springs M, one end of each spring being fastened to the screws C. These screws work in a slot cut in the upper part of S, and are connected to the adjusting screw E. When E is turned the screws are raised or lowered accordingly, and also the balls on the arms D.
Connection is made with the contact springs S, S', by means of the springs T, T', which press against the spindles J, J'.
There are several methods of synchronising that are in constant use in high-speed telegraphy, in which the limit of error is reduced to a minimum, and some modification of these methods will perhaps solve the problem, but it must be remembered that synchronism is far easier to obtain where the two stations are connected by a length of line than where the two stations are running independently.
In one system of ordinary photo-telegraphy synchronism is obtained in the following manner. The receiving cylinder travels at a speed slightly in excess of the transmitting cylinder, and as its revolution is finished first is prevented from revolving by a check, and when in this position the receiving apparatus is thrown out of circuit and an electro-magnet which operates the check is switched in. When the transmitting cylinder has completed its revolution the transmitting apparatus, by means of a special arrangement, is thrown out of circuit for a period, just long enough for a powerful current to be sent through the line. This current actuates the electro-magnet. The check is withdrawn and the receiving cylinder commences a fresh revolution in perfect synchronism with the transmitting cylinder. As soon as the check is withdrawn the receiving apparatus is again placed in circuit until another revolution is completed. As the receiver cannot stop and start abruptly at the end of each revolution a spring clutch is inserted between the driving motor and the machine.
Although a method of synchronising similar to this may later on be devised for wireless photography, the writer, from the result of his own experiments, is led to believe that results good enough for all practical purposes can be obtained by fitting a synchronising device whereby the two machines are started work at the same instant, and relying upon the perfect regulation of the speed of the motors for correct working.
The method of isochronism must, however, be nearly perfect in its action, as it is easy to see that with only a very slight difference in the speed of either machine this error will, when multiplied by 40 or 50 revolutions, completely destroy the received picture for practical purposes.
From what has been written in this and in the preceding chapters it will be evident that the successful solution of transmitting photographs by wireless methods will necessitate the use of a great many pieces of apparatus all requiring delicate adjustment, and depending largely upon each other for efficient working. As previously stated, there is at present no real system of wireless photography, the whole science being in a purely experimental stage, but already Professor Korn has succeeded in transmitting photographs between Berlin and Paris, a distance of over 700 miles. If such a distance could be worked over successfully, there is no reason to doubt that before long we shall be able to receive pictures from America with as great reliability and precision as we now receive messages.
In nearly all wireless photographic systems devised up to the present the chief portion of the receiver consists of a very sensitive galvanometer, and although very good results have been obtained by their use they are more or less a nuisance, as the extreme delicacy of their construction renders them liable to a lot of unnecessary movement caused by external disturbances. A galvanometer of the De' Arsonval pattern, used by the writer, was constantly being disturbed by merely walking about the room, although placed upon a fairly substantial table; and for the same reason it was impossible to attempt to place the driving motor of the machine on the same table as the galvanometer. For ship-board work it will be evident that the use of such a sensitive instrument presents a great difficulty to successful working, and a good opening exists for some piece of apparatus--to take the place of the galvanometer--that will be as sensitive in its action but more robust in its construction.
THE "TELEPHOGRAPH"
Free books android app tbrJar TBR JAR Read Free books online gutenberg
(0)More posts by @FreeBooks
