With this in mind, Lieut. Finch turned to radio facsimile in 1921, when that art was still in the laboratory stage. After years of. persistent work—hampered by thievery and espionage—he finally turned out a practical instrument, the latest developments of which have been financed by the United States government. Facsimile units now undergoing field tests for military usage are approximately typewriter size and weigh less than 25 pounds. Each unit is both receiver and transmitter, Out from the receiver roll pictures 8% by 7 inches.
Absolute secrecy in transmission is obtained by a device which delicately synchronizes sending and receiving sets. Imagine two small drums, miles apart, revolving at the same speed without a thousandth of a second variation. Every time the transmitting drum makes one exact revolution it sends out by radio a distinctive synchronizing tone, and that tone travelling by radio controls each revolution of the receiving drum. No receiving machine can pick up more than indecipherable lines, unless it is set upon that particular frequency or pitch. And if all friendly machines change tone and wave length, say, every fifteen minutes by prearrangement, an enemy wouldn't have a chance of hitting on the proper frequency.
Of special interest to the General Staffs is facsimile air reconnaissance. Heretofore the great problem has been not to get information but to bring it back intact. Now, during hazard-Ous reconnaissance raids, observers may send back a steady stream of notated maps or photographs developed in the plane right up to the second they're shot down behind the enemy's line.
Text read from the printed page and cleaned; old type leaves the occasional error.