Polygraph (duplicating device)

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For other uses, see Polygraph (disambiguation).
Reproduction of Jefferson's polygraph at the Smithsonian Institution
One of the polygraphs used by Thomas Jefferson, a portable version

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 in 1803, it was most famously used by the third U.S. president, Thomas Jefferson, who acquired his first polygraph in 1804, later suggesting improvements to Charles Willson Peale, owner of the American rights. As a prolific letter writer, the preservation of Jefferson's copies have offered historians extensive insights into Jefferson's viewpoints and actions.[1] Jefferson called the polygraph "the finest invention of the present age".[2][3] A description of Jefferson's office routine in his own words may be read here.[4]

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.[5]

The quality of the copy made by a machine in good condition is so fine that it is considered by some[6] 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.[6]

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.

Description of the device

Annotated engraving of Hawkin's Polygraph from Rees's Cyclopædia, ca. 1820

For a QuickTime animation see.[7]

Stationary parts

Platform

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.

Bridge

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.

Inkwells

Inkwells are provided beneath the bridge for each pen at corresponding locations. The dipping of the master pen will thus re-ink the slave pen.

Moving parts

The device consists of two pens transmitting motion in five degrees of freedom through four interlinked mechanisms:

  1. A horizontal pantograph maintains identical planar (X and Y axis) movement, with two degrees of freedom
  2. An angled pantograph descending from the bridge maintains identical vertical (Z axis) movement
  3. A torsion beam maintains identical pen fore-and-aft tilt
  4. A parallel linkage maintains identical pen side-to-side tilt

In addition, a vertical suspension spring balances the weight of the moving parts.

Planar pantograph

The range of the planar mechanism must be sufficient to encompass the papers (individually) being written upon and allow access for each pen to its respective inkwell.

A simple pantograph is used to translate the planar motion of one pen to the other. The pantograph consists of two complete variable parallelograms ("d" and "e" on diagram):

  • Base parallelogram: The base parallelogram is attached to two fixed pivot points at the far side of the base plate. The pivoting arms are supported at their midpoints by wheels which rest on the base and this part remains in a plane parallel to the base.
  • Extension parallelogram: The extension parallelogram is attached to the base parallelogram by pivots that allow the pen-side edge to be lifted away from the base.

Descending pantograph

A second pair of parallelogram links maintains vertical correspondence between the two pens ("D" and "E" on diagram). These consist of two variable parallelogram frames attached at a common edge, one of which is also attached to the bridge, while the other is attached to one of the pen supports at each corner. The projecting sides of the upper frame consist of parallel linkages.

Pen lift transfer

The combination of forces between the horizontal and vertical enables the pen lift of the principal pen grasped by the user from the paper to be transferred to the copying pen.

  • Pen mounts: Each pen is mounted in such a way that it allows the pen to be used at various angles, necessary to allow a normal writing style. The pens are offset from their pivots by a short stem that projects at right angles from below the midpoint of the pen, allowing the pen to be grasped in whatever way is usual to the user. ("a" on diagram)
  • Pen tilt (fore and aft): A torsion beam joins the pen mounts to maintain correspondence of tilt with respect to the direction away from the user. ("G" on diagram)
  • Pen tilt (side-to-side): A parallel linkage maintains correspondence of tilt with respect to a direction parallel to the front face of the device. ("H" on diagram)
  • Suspension beam and spring: At the center of the bridge a beam projects horizontally toward the user ("K" on diagram), from which a vertical suspension spring ("I" on diagram)balances most of the weight of the moving parts so that the user is neither fatigued by using the device nor is required to adjust their writing style.

Viewing in museums

Original polygraphs may be viewed at a number of locations around the world.

Some United States locations:

A modern version

The Griffin Discovery Room at Monticello contains a simplified and durable version, intended for use by children. An image of this version in use may be seen here.

See also

  • LongPen, a remote signing device conceived by writer Margaret Atwood

References

  1. ↑ The Works of Thomas Jefferson, 12 vols. At The Online Library of Liberty (website of Liberty Fund, Inc.)
  2. ↑ Some information and quotation from curator's card, Smithsonian Museum of American History
  3. ↑ Letter: Thomas Jefferson to James Bowdoin, July 10, 1806, Washington – Writings of Jefferson 6:7 The full line is "I think it the finest invention of the present age and so much superior to the copying machine that the latter will never be continued a day by any one who tries the polygraph". The "copying machine" refers the copying press, which enabled a copy to be made by writing the original using special transferable inks and damp papers to receive the copies. (quoted and described in Bedini, p. 147.)
  4. ↑ "Drudging at the Writing Table" (Monticello.org)
  5. ↑ Bedini, Silvio A. (1984). Thomas Jefferson and His Copying Machines. Charlottesville: University Press of Virginia. ISBN 978-0-8139-1025-3, p. 53 et al.
  6. ↑ 6.0 6.1 Thomas Jefferson and the Polygraph (The Raab Collection, monograph concerning difficulty of authenticating originals vs. copies in Jefferson's letters: "Textual anomalies have not historically been considered as indicators")
  7. ↑ Polygraph movie (requires an appropriate browser plug-in and browser setting) To use this animation, place the cursor on the image, click, and drag. The pens will follow your movement in the writing plane, but pen lift and the pen tilts are not demonstrated.

Further reading

  • Bedini, Silvio A. (1984). Thomas Jefferson and His Copying Machines. Charlottesville: University Press of Virginia. ISBN 978-0-8139-1025-3

External links

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