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Teledeltos was a dry, electro-sensitive recording paper developed for facsimile telegram systems. Its ability to reveal a dark backing when marked with an electrically energized stylus later made it useful for resistor prototyping and demonstrations of electrical fields. That was an inventive second use—not evidence that the paper itself caused a revolution.
What Teledeltos paper was
Teledeltos combined a light-gray electro-sensitive coating on one side with a carbon-black backing. Applying current through a stylus burned away the surface coating, exposing the dark layer and leaving a visible mark. This gave communications equipment a way to record an image on dry paper.
A 2015 Hackaday account says the paper could be used with either AC or DC. It reports that Western Union technicians had better results with a positive stylus and DC; that is a reported operating preference, not a universal rule for every setup.
How it fits into the history of facsimile
Conductive paper did not begin with Teledeltos. Smithsonian Education says Scottish clockmaker Alexander Bain devised a facsimile system in 1842 that transferred an image from one electrically conductive sheet to another, and patented what it calls the “automatic electrochemical recording telegraph” in 1843. Those dates come from the Smithsonian’s 1998 educational account, not a review of the original patent text. Smithsonian Education’s account of Bain
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- Black Conductive Paper (Pack of 25)
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Western Union began developing Teledeltos in the 1930s to simplify facsimile telegram transmission, according to the Hackaday history. A January 1949 Western Union Technical Revue search result also identifies Teledeltos as dry recording paper and mentions its H and L grades. Because the scan was not available in full, details beyond that indexed description should be treated cautiously. Western Union Technical Revue archive
Why it had two resistance grades
Teledeltos was sold in “L” low-resistance and “H” high-resistance varieties. Hackaday attributes differences in resistivity to the quality of conductive fibers and how they were processed and distributed during papermaking. The grades mattered because the paper’s electrical behavior was part of how it recorded marks and, later, how it could be used as an analog modeling surface.
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- Suitable for laboratory fuel cells and various electrochemical systems, offering stable structure, good electrical conductivity, and excellent gas diffusion performance
- Dimensions: 20×21 cm, thickness: 0.20±0.01 mm, featuring good tensile and bending properties for easy assembly and use
- Hydrophilic design with good wettability on the carbon paper surface, for applications requiring liquid permeation, such as electrochemical deposition and solution-phase synthesis
- Resistance ≤ 11 mΩ·cm², effectively reducing resistive losses for efficient electron transfer
- Porosity of approximately 80%, facilitating rapid transport and uniform distribution of reactant gases, meeting the requirements for gas diffusion layer applications
What engineers did with it after facsimile
The paper’s most interesting second life came from treating it as a physical way to explore electrical behavior. Hackaday describes linear integrated-circuit engineer Bob Widlar using Teledeltos to try configurations for unusually shaped ballast resistors. The same account says resistance paper could be used to make one-off standard or variable resistors, demonstrate current flow, model electric potentials, and illustrate electromagnetic fields.
These are related but distinct jobs. A facsimile system used the material to make a visible recording mark; an engineering demonstration used its resistive surface to make electrical behavior easier to observe or model. A 1952 Western Union Technical Revue search result reported that 50 Desk-Fax transceivers entered Newark offices for field trial in 1948 and that installations had reached 26 cities by late 1951. Those are figures published by Western Union in 1952 and surfaced in an indexed result; the scan was not available in full for independent checking. Western Union Technical Revue archive
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Teledeltos compared with modern paper circuits
Modern paper circuits are a useful classroom comparison, but they are not Teledeltos experiments. NASA Chandra’s activity uses ordinary paper, copper conductive tape, LEDs, and a small battery to build a simple low-voltage circuit. Its page describes the activity this way: “Paper circuits function as simple low-voltage electronic circuits.” NASA Chandra paper-circuit activity
| Use | Material and purpose |
|---|---|
| Teledeltos facsimile recording | Electro-sensitive paper with a carbon-black backing; a stylus mark reveals the dark layer. |
| Resistance-paper demonstrations | Teledeltos or related resistance paper used to explore resistors, current flow, or electrical potentials. |
| Modern paper circuits | Ordinary paper with copper conductive tape, LEDs, and a small battery; intended to teach basic low-voltage circuit continuity. |
NASA Chandra recommends adult supervision for younger children because the small battery presents a choking hazard. Its activity is a practical present-day way to introduce circuits, but it should not be presented as a substitute for Teledeltos when the goal is resistance-paper modeling.
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- Carbon paper as a whole is hydrophobic, and its hydrophilicity is relatively hydrophobic.
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