COPYING

MACHINES  . Appliances of various kinds have been devised for producing copies of writings made by the pen or pencil . A simple method commonly adopted when only a single copy is required is to write the original with specially prepared copying ink (formed by adding some thickening substance like sugar or gum to ordinary ink), to place upon it a damped sheet of thin absorbent paper, and to press the two together in some way, as in a copying press . The resulting impression, being reversed, must be read from the back of the absorbent paper, which is thin enough to be transparent . Another process, by which a considerable number of copies can be made simultaneously, consists in interleaving a number of sheets of thin white paper with sheets of paper prepared with lampblack (" carbon paper ") and writing on the top sheet with a " style " or other sharp-pointed instrument, The hectograph may be taken as typical of manifolding processes analogous to lithography . In it the writing is in first instance done with aniline ink, and then a transfer is made to a plate of a gelatinous composition, from which a series of duplicates can be taken off . Another class of methods involves the preparation of what are essentially stencils . In the cyclostyle, paper of a special kind is stretched over a smooth metal plate, and the writing instrument consists of a holder having at the end a small wheel provided with a serrated edge on its periphery, which perforates the paper with lines of minute cuts and thus forms a stencil . When ink is passed over this stencil with a roller it goes through the perforations and leaves an impression on a piece of paper placed underneath . In the trypograph a similar result is attained by using a simple style for writing, but stretching the paper over a metal plate having its surface covered with fine sharp corrugations which pierce the paper as the style is moved over them . In the Edison electric pen the stencil is formed by the aid of a style containing a fine needle, which is rapidly moved up and down by a .small electric motor mounted at the top of the pen, and thus a series of minute holes is punctured in the paper by the act of writing . For copying plans and drawings, engineers, architects, &c., use a " blue print " process which depends on the action of light on certain salts of iron (see SUN-COPYING and PHOTOGRAPHY) .