Polygraph (duplicating device)

A Polygraph is a device that produces a copy of a piece of writing simultaneously with the creation of the original, using pens and ink.

Patented by John Isaac Hawkins on May 17, 1803, it was most famously used by the third U.S. president, Thomas Jefferson, who acquired his first polygraph in 1804 and later suggested improvements to Charles Willson Peale, owner of the American rights. Because Jefferson was a prolific letter writer, the preservation of his copies have offered historians extensive insights into Jefferson's viewpoints and actions. Jefferson called the polygraph "the finest invention of the present age". A description of Jefferson's office routine in his own words may be read here.

Another American, Benjamin Henry Latrobe was the first customer of Peale's and not only introduced the device to Jefferson but was also instrumental in its improvement.

The quality of the copy made by a machine in good condition is so fine that it is considered by some that only characteristics other than the quality of the script (e.g., having been mailed, etc.) are reliable indicators (under the assumption that the original was mailed and not the copy), although a worn or maladjusted machine will produce specific indications in the copy.

Mechanisms of this type are more generally known as pantographs, which include simple devices for making copies at various enlargements or reductions by tracing over a drawing.

For a QuickTime animation see.

The platform forms both a writing surface (in part) and a base and reference plane for the moving and other stationary parts.("F" on diagram) In a portable device the platform is in two parts that form the covers of the enclosure formed for transport. As suggested by Jefferson, a non-portable version uses a single board for the entire platform.

Also known as a "gallows frame", the bridge spans the platform ("A" and "B" on diagram). In the non-portable device this is mounted on posts permanently attached to the platform. In the portable version the bridge, side posts, and a bottom cross piece form the outer rim of enclosure when in the state for transportation.

The bridge divides the platform into two portions, the part toward the user upon which the papers are placed (see illustration above), and the stationary part away from the user that contains a portion of the planar pantograph. The function of the bridge is to form a base for the vertical movement linkage and a support for the suspension spring support beam.

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