The wireless transmission of photographs is, on the other hand, of quite recent growth, the first practicable attempt being made by Mr. Hans Knudsen in 1908. It may seem rather premature to talk about the wireless transmission at a time when the systems for transmitting over ordinary conductors are not perfectly developed, but everything points to the fact that for long-distance transmission a reliable wireless system will prove to be both cheaper and quicker than transmission over ordinary land lines and cables.
The effects of capacity and inductance—properties inherent to all telegraph systems using metallic conductors—have a distinct bearing upon the two questions, how far and how quickly can
photographs be transmitted? Owing to the small currents received and to prevent interference from earth currents it is necessary to use a complete metallic circuit. If an overhead line could be employed no difficulty would be experienced in working a distance of over 1000 miles, but a line of this length is impossible—at least in this country—and if transmission is attempted with any other country, a certain amount of submarine cable is essential. It has been found that the electrostatic capacity of one mile of submarine cable is equal to the capacity of 20 miles of overhead line, and as the effect of capacity is to retard the current and reduce the speed of working, it is evident that where there is any great length of cable in the circuit the distance of possible transmission is enormously reduced.
If we take for an example the London-Paris telephone line with a length of 311 miles and a capacity of 10.62 microfarads, we find that about half this capacity, or 5.9 microfarads,[[1]] is contributed by the 23 miles of cable connecting England with France.
In practice the reduction of speed due to capacity has, to a great extent, been overcome by means of apparatus known as a line-balancer, which hastens the slow discharge of the line and
allows each current sent out from the transmitter—the current in several systems being intermittent—to be recorded separately on the receiver. Photographs suitable for press work can now be sent over a line which includes only a short length of cable for a distance of quite 400 miles in about ten minutes, the time, of course, depending upon the size of the photograph. In extending the working to other countries where there is need for a great length of cable, as between England and Ireland, or America, the retardation due to capacity is very great. On a cable joining this country with America the current is retarded four-tenths of a second. In submarine telegraphy use is made of only one cable with an earth return, but special means have had to be adopted to overcome interference from earth currents, as the enormous cost prohibits the laying of a second cable to provide a complete metallic circuit. The current available at the cable ends for receiving is very small, being only 1/200000th part of an ampere, and this necessitates the use of apparatus of a very sensitive character. One system of photo-telegraphy in use at the present time, employs what is known as an electrolytic receiver (see Chapter III.) which can record signals over a length of line in which the capacity effects are very slight, with the marvellous speed of 12,000 a minute, but this speed rapidly decreases with an increase of distance between the
There have been numerous suggestions put forward for the wireless transmission of photographs, but they are all more or less impracticable. One of the earliest systems was devised by de' Bernochi of Turin, but his system can only be regarded interesting from an historical point of view, and as in all probability it could only have been made to work over a distance of a few hundred yards it is of no practical value. Fig. 2 will help to explain the apparatus. A glass cylinder A' is fastened at one end to a threaded steel shaft, which runs in two bearings, one bearing having an internal thread corresponding with that on the