Electrolytic facsimile printing is a technique for producing facsimile images from wire or radio transmissions. The concept was discovered as early as 1843 by the British inventor Alexander Bain. The facsimile printing machine contains two metal devices that receive the electrical current from the transmission. A roll of wet paper is fed between the metal devices and a reaction occurs between the metal and a compound impregnated in the paper. The reaction forms a stain similar in chemical composition to an ink or a dye. The electrical current varies according the density of the original image, which in turn causes the strength of the chemical reaction in the facsimile printer to vary and reproduce the same image densities. Electrolytic facsimile prints are often on a slightly translucent paper, and the image color can range from warm brown to black. The image will always be readily visible on the back of the print.
The Associated Press employed this technology in their Photofax (TM) wire transfer facsimile printer, which was introduced in 1955. The process was also used by other news agencies, and by weather stations to print weather maps. Any image printed off one of these devices contains a strip of printed text describing the image, listing the source, and identifying the date of the image and the story. The facsimile print functioned as an intermediary in the news process; images printed off the machines in newsrooms continuously through the day and editors would choose images to reproduce in their newspapers. Any image chosen would then need to be made into a half-tone print to facilitate ink printing.