The telautograph's invention is attributed to the American engineer
Elisha Gray, who patented it on July 31, 1888. Gray's patent stated that the telautograph would allow "one to transmit his own handwriting to a distant point over a two-wire circuit." It was the first facsimile machine in which the stylus was controlled by horizontal and vertical bars. The telautograph was first publicly exhibited at the 1893
World's Columbian Exposition held in
Chicago. Gray started experimenting in 1887 with analog transmission of the pen position signals using
variable resistances as was done in previous devices, but was dissatisfied with the performance he achieved. He then turned to pulse-based or digital pen position transmission. Gray's early patents show devices to accomplish the required functions over two line wire circuits with a common ground connection. Pulses were sent over each wire to signal small steps of pen movement. Momentary current interruptions of a baseline
direct current signaled pen lifting/lowering and paper feed, and changing polarities were used to encode pen movement direction. While the patent schema's geometry implies vertical and horizontal coordinates, Gray's first practical system (discussed later) had a different coordinate scheme, based on transmitting two radial distances along approximately diagonal directions from two fixed points. Later systems used in the 20th century transmitted the angle of two crank arm joints in a
five bar linkage, comprising two pen motor cranks, two pen linkage bars, and the body of the instrument. In an 1888 interview in
The Manufacturer & Builder (Vol. 24: No. 4: pages 85–86) Gray said: By my invention you can sit down in your office in Chicago, take a pencil in your hand, write a message to me, and as your pencil moves, a pencil here in my laboratory moves simultaneously, and forms the same letters and words in the same way. What you write in Chicago is instantly reproduced here in fac-simile. You may write in any language, use a code or cipher, no matter, a fac-simile is produced here. If you want to draw a picture it is the same, the picture is reproduced here. The artist of your newspaper can, by this device, telegraph his pictures of a railway wreck or other occurrences just as a reporter telegraphs his description in words. However these first devices were crude to the point of uselessness. Some of his subsequent refinements changed the encoding scheme. They also mention use of four wires for increased speed and accuracy, but the additional wires were later abandoned. It's clear from the commentary in these and other patents that Gray needed to increase the speed and accuracy of his pulse based system, and in fact he patented a large number of increasingly complicated and refined mechanisms to achieve this. In 1893 Gray's system using the mechanism seen in Pat. US491347 An article in Manufacturer and Builder of this year describes the current and previous versions. shows the rapidity with which an operator might move the pen. This type of use would produce perhaps 600-1000 pulses per second on a digital system, a challenge for any electromechanical system connected over earth return telephone/telegraph lines. A more elegant technology was around the corner, and an analog coup was being staged at the turn of the century. By the end of the 19th century the telautograph was modified Calling it the telewriter, Ritchie's version of the telautograph could be used for either copying or speaking over the same telephone connection. Ritchie had returned to the analog principle and made it work well. He did this by adding an
AC signal whenever the pen needed to be lowered, on top of the direct current position signal already on the line wires. The angle of the two pen
crank bars was turned into the position signal by two
rheostats, driving large
D'Arsonval movements at the receiver that moved similar crank bars, in turn moving the receiver pen. Interruption of the direct current advanced the paper. The AC pen lowering signal was highly important. If Ritchie understood the significance of this technique, he strangely failed to reveal (or protect) this principle in his patents. George S. Tiffany on behalf of the Gray National Telautograph Company understood the significance of the AC signal quite well. In the patent he filed shortly after and presumably in response to Ritchie he explains that the use of either an AC signal superimposed on the pen current signal or intentional mechanical vibrations added at the receiver can overcome static pen and actuator friction, and allow the pen to follow the transmitter quite perfectly. This principle is in common use today in the form of
dither, as applied to proportional pneumatic and hydraulic control valves and regulators. A dither signal can overcome both magnetic hysteresis and static friction and was preferable to mechanical vibration, as later Telautograph designs used it exclusively. Apparently this technique worked well, because even though Tiffany studiously avoided every constructional feature of Ritchie's patent, he used the exact same fundamental technique, and the analog telautograph principle continued to be used for at least the next 35 years, also see interior view. Tiffany patents after 1901 refined the mechanism but not the principle. Ritchie marketed his design as the Telewriter in the UK. it's not clear if this was ever commercialized. ==Usage==