Editing Optical telegraph (section)

====Edelcrantz system technical operation====
After some initial experiments with Chappe-style indicator arms, Edelcrantz settled on a design with ten iron shutters. Nine of these represented a 3-digit octal number and the tenth, when closed, meant the code number should be preceded by "A". This gave 1,024 codepoints which were decoded to letters, words or phrases via a codebook.<ref>Holzmann & Pehrson, p. 103</ref> The telegraph had a sophisticated control panel which allowed the next symbol to be prepared while waiting for the previous symbol to be repeated on the next station down the line. The control panel was connected by strings to the shutters. When ready to transmit, all the shutters were set at the same time with the press of a footpedal.<ref name="Holzmann & Pehrson, pp. 104–105"/>

The shutters were painted matte black to avoid reflection from sunlight and the frame and arms supporting the shutters were painted white or red for best contrast.<ref>Edelcrantz, p. 164</ref> Around 1809 Edelcrantz introduced an updated design. The frame around the shutters was dispensed with leaving a simpler, more visible, structure of just the arms with the indicator panels on the end of them. The "A" shutter was reduced to the same size as the other shutters and offset to one side to indicate which side was the [[most significant digit]] (whether the codepoint is read left-to-right or right-to-left is different for the two adjacent stations depending on which side they are on).<ref name="Holzmann & Pehrson, p. 117"/> This was previously indicated with a stationary indicator fixed to the side of the frame, but without a frame this was no longer possible.<ref>Edelcrantz, pp. 144, 146</ref>

The distance that a station could transmit depended on the size of the shutters and the power of the telescope being used to observe them. The smallest object visible to the human eye is one that subtends an angle of 40 [[seconds of arc]], but Edelcrantz used a figure of 4 [[minutes of arc]] to account for atmospheric disturbances and imperfections of the telescope. On that basis, and with a 32× telescope, Edelcrantz specified shutter sizes ranging from 9 inches ({{nowrap|22 cm}}) for a distance of half a [[Swedish mile]] ({{nowrap|5.3 km}}) to 54 inches ({{nowrap|134 cm}}) for 3 Swedish miles ({{nowrap|32 km}}).<ref>Edelcrantz, pp. 166–167</ref> These figures were for the original design with square shutters. The open design of 1809 had long oblong shutters which Edelcrantz thought was more visible.<ref>Edelcrantz, p. 165</ref> Distances much further than these would require impractically high towers to overcome the curvature of the Earth as well as large shutters. Edelcrantz kept the distance between stations under 2 Swedish miles ({{nowrap|21 km}}) except where large bodies of water made it unavoidable.<ref>Edelcrantz, p. 169</ref>

The Swedish telegraph was capable of being used at night with lamps. On smaller stations lamps were placed behind the shutters so that they became visible when the shutter was opened. For larger stations, this was impractical. Instead, a separate tin box matrix with glass windows was installed below the daytime shutters. The lamps inside the tin box could be uncovered by pulling strings in the same way the daytime shutters were operated. Windows on both sides of the box allowed the lamps to be seen by both the upstream and downstream adjacent stations. The codepoints used at night were the [[diminished radix complement|complements]] of the codepoints used during the day. This made the pattern of lamps in open shutters at night the same as the pattern of closed shutters in daytime.<ref>Edelcrantz, pp. 170–171</ref>