A chart recorder is an electromechanical device that records an electrical or mechanical input trend onto a piece of paper (the chart). Chart recorders may record several inputs using different color pens and may record onto strip charts or circular charts. Chart recorders may be entirely mechanical with clockwork mechanisms, electro-mechanical with an electrical clockwork mechanism for driving the chart (with mechanical or pressure inputs), or entirely electronic with no mechanical components at all (a virtual chart recorder). Chart recorders are built in three primary formats. Strip chart recorders have a long strip of paper that is ejected out of the recorder. Circular chart recorders have a rotating disc of paper that must be replaced more often, but are more compact and amenable to being enclosed behind glass. Roll chart recorders are similar to strip chart recorders except that the recorded data is stored on a round roll, and the unit is usually fully enclosed. Chart recorders pre-dated electronic data loggers which have replaced them in many applications. Origins Charles Babbage incorporated a chart recorder into the dynamometer car that he built in 1838 or 1839. Here is how he described it: "A roll of paper a thousand feet in length was slowly unwinding itself upon the long table ... About a dozen pens connected with a bridge crossing the middle of the table were each marking its own independent curve gradually or by jumps ..." The paper advance was geared to the wheels of the railroad carriage, while pens recorded time, the drawbar pull of the locomotive, and numerous other variables. Part of Samuel Morse's telegraph system was an automatic recorder of the dots and dashes of the code, inscribed on a paper tape by a pen moved by an electromagnet, with a clockwork mechanism advancing the paper. In 1848-1850 a system of such registers was used by John Locke to improve the precision of astronomical observations of stars, providing timing precision much greater than previous methods. This method was adopted by astronomers in other countries as well. William Thomson, 1st Baron Kelvin's syphon recorder of 1858 was a sensitive instrument that provided a permanent record of telegraph signals through long underwater telegraph cables. These recorders came to be referred to as pen registers, although this term later became part of law enforcement jargon referring to the use of such a register to record dialed telephone numbers. A patent