The Conductive Paper That Sparked A Revolution

Legendary electrical engineer and linear IC trailblazer Bob Widlar was just like you. What I mean is that he would use everything available to him to mock up circuits, create prototypes, and make things work. One of the simplest and coolest tools he used was a conductive paper called Teledeltos. This wonderful stuff allowed him to define and test various configurations for the oddly-shaped ballast resistors he used in some of his high-performance circuit designs. But it wasn’t created for people like you and Bob. Teledeltos paper was created and trademarked by communications giant Western Union to drastically improve the convenience of telegrams.

Development of the electric telegraph ushered in the era of global communication. Suddenly, people could send messages to the other side of the world in a fraction of the time it took by post. The telegraph absolutely revolutionized human communication. It was the e-mail and the Twitter of its time. The telegraph’s efficiency made the Pony Express pretty much obsolete by the 1860s. And for a very long time it was much cheaper for people to send a telegram than make a long-distance phone call.

The Advantages of Facsimile

Translated from ancient Greek, ‘teledeltos’ basically means writing tablet at a distance. Western Union began developing Teledeltos paper in the 1930s for the purpose of transmitting telegrams by facsimile, a method that would greatly reduce the time it took to input messages into the system and get them out on the other side. As long as both the sender and the receiver had facsimile machines, a handwritten telegram could be transmitted without having to be typed by a clerk or translated into code. Teledeltos paper was also used in a variety of chart recorders, like seismographs and map plotters. The ability to feed a handwritten message, a photograph, or a map of enemy territory into a machine that transmitted an exact copy was a real game changer.

Because of its composition, Teledeltos paper could be easily marked without an electrolyte. It marked so well that photographs and other graphic information could be transmitted, and no processing was required on the receiving end. A dry recording paper is also much less sensitive to light and to temperature extremes. More importantly, properly stored dry paper is impervious to fungal growth. Teledeltos paper could sit around indefinitely without becoming useless. The only real disadvantage to this type of paper was the somewhat laborious process that went into achieving the desired resistance. Fax machines eventually moved on to digital transmission and thermal printing technology.

Image credit: MIT

Sparking a Revolution

Teledeltos paper has a light gray electro-sensitive coating on one side, and the other side is carbon black. When a current is applied with a stylus to the coated side of the paper, the coating is instantly burned away, revealing the carbon black. Teledeltos paper could be marked using either AC or DC. Polarity didn’t matter, either, but the boys in the lab at Western Union had better luck when they used a positive stylus with DC rather than a negative one.

Teledeltos paper was made in two types—“L” for low resistance and “H” for high. The resistivity of a roll of Teledeltos paper depended on the quality of the conductive fibers that went into it. The paper’s electrical characteristics were also influenced by the fiber beating process and the distribution of the conductive fibers by the supercalender, a system of hard rollers used in papermaking and other processes that press and smooth paper and other materials to increase the density.

Teledeltos to the Rescue

The Western Union Telecar printed telegrams on the go and delivered them to homes and businesses.
The Western Union Telecar printed telegrams on the go and delivered them to homes and businesses. Image credit: Modern Mechanix

Western Union was eager to extend its reach into private businesses and public places so that patrons who weren’t heavy telegram users didn’t have to visit a telegram office in order to share a bit of good news or to send their condolences. The company’s Telefax division came up with several types of machines to serve different business needs.

Some messages continued to be delivered by hand, but they weren’t printed at the central office. Western Union created a Telecar service to print telegrams transmitted to the car by the central office and deliver them to people’s homes. Messages were printed onto recording blanks that were cut automatically by a Telefax recorder situated in the car’s passenger area. The Telecar’s radio and amplification equipment was in the trunk.

The standard Telefax machine for office use was fairly large, like an early microwave oven. A smaller version called the DeskFax was only about the size of a breadbox, and these units occupied the desks of many businessmen and secretaries because of their convenience.

A Western Union DeskFax unit. Image from [B. Hilpert]
A Western Union DeskFax unit. Image from [B. Hilpert]
Both the Telefax and the DeskFax scanned and recorded telegrams using a rotary drum mechanism. A message could either be typed or handwritten onto a telegram blank. The sender then wrapped the telegram around a drum and set the machine to send. The machine would scan the message optically and then transmit it to the central office.

Before sending it on to the recipient, an attendant at the telegram office had to remove the incoming message and wrap it around the drum of a transmitting machine. Once connected to the receiving party’s line, the far end unit would buzz to arouse attention. The receiving patron would then load a blank on to their DeskFax’s roller and set their machine to receive.

Teledeltos for Hacking and Education

Measuring potential differences. Image from
Measuring potential differences.
Image from UCSD

Conductive paper like Teledeltos has many applications aside from fax machines and Fathometers. For starters, it’s great for making one-offs of both standard and variable resistors. Conductive paint can be used as connection points for wires. The paper is also well-suited for simulating current flow through circuits using a fraction of the current intended in production. Vacuum tube designers used Teledeltos for modeling potentials. Teledeltos can also be used to visualize electromagnetic potentials and perform field plotting.

We’re sure that at least a few of our readers out there used Teledeltos or something like it in school or on the job. Did you know you can still buy it? Teledeltos paper itself is still available from two companies in the UK, Better Equipped and Timstar. In the US, you can get it from Pasco in packs of 50 and 100 sheets, with and without a grid pattern.

[Teledeltos paper image is a product photo from Better Equipped]

26 thoughts on “The Conductive Paper That Sparked A Revolution

  1. I remember hacking one of those DeskFax units to receive weather maps over HF radio back in the 60s.

    Start by winding your own coils to make a demodulator…

    I remember the way it actually burned the black into the paper. These days, it would probably have to have an exhaust fan to outside.

  2. I love Teledeltos!

    In my book “Design and Development of Medical Electronic Instrumentation” (published by Wiley in 2005 – pardon the shameless plug), I described how I use Teledeltos for solving difficult forward and inverse problems in biomedical engineering:

    “Probably the simplest physical models used to shed light on the inverse problem in electrocardiography
    were two-dimensional models. Grayzel and Lizzi [1967] used conductive
    paper (Teledeltos) to create two-dimensional inhomogeneous models of the human thorax,
    to which they attached current source–sink pairs (bipoles) to represent the heart. Teledeltos
    paper is a resistive paper that has uniform resistance. As shown in Figure 6.32, Teledeltos
    paper can simply be cut, after suitable scaling, to the shape of the body region to be investigated.
    Then electrodes are painted on the paper with conductive silver ink so that sources
    (voltage and current sources, and sinks or loads) can be attached to the paper at the appropriate
    places to set up an analog of the boundary conditions desired. The extent and value
    of inhomogeneities are controlled by means of perforations or silver spots applied to the
    conductive paper. The ratio of hole diameter to hole spacing determines the relative
    increase in resistivity of the area punched. The ratio of silver-dot diameter to center spacing
    determines the relative decrease in resistivity.

    To find solutions, one simply reads out the field intensity values by using a sharp-tipped
    voltmeter probe applied to the paper at any point that field intensity is desired. Thus, a very
    simple laboratory setup can be used to solve sets of complicated partial differential equations
    empirically without the user even knowing what partial differential equations he or
    she is actually solving!

    … The text in the book continues wit the way its done in practice and the math necessary for interpretation (pages 292 – 296).

    Kudos Kristina for showcasing this “ancient” technology! There are many engineering problems which can be solved more easily and cost-effectively with Teledeltos than with a computer simulation.



    1. Your book sounded so interesting I wanted to read it but unfortunately as with all good books on such out of the way topics its price point puts it out of the casual reading spectrum :(

  3. there were a few printers that used this paper, one for the TRS-80 and another made by Anadex.

    the TRS-80 printer was a bitch if you were reading or writing to disk at the same time you were printing.

      1. The American version used the Alphacom printer, which was thermal. Seemed like it had several heater heads, spaced out about an inch apart, and the mechanism would reciprocate, slide side to side. Applying a constant voltage to them gave a wiggly pattern.

        The British one used aluminium-coated paper with a black backing underneath, burned away with a spark. Running straight off the computer’s expansion bus, including drawing power. How the spark didn’t wreck every signal seems like a miracle to me.

        So was ZX Printer paper the same as Teledeltos? I don’t think it was resistive, except in the sense all conductors are.

    1. I had an FX-702P and its tiny printer, the FP-10 was using metallic paper. The printouts should still be readable today, unlike those of the thermal printers that followed.
      On a somewhat related subject, does anyone knows where to find BLUE thermal paper? It seems to have been wiped from the face of the Earth.

        1. You can always cut it down. The FX-702P was one of those calculator-type tiny computers, one-line LCD display and teeny keys. Some people collect them, I’d love to, no idea what they were ever used for. So the printer used till-roll sized paper.

  4. People usually don’t think of Western Union when they think of innovation but the company was responsible for many technical innovations right up into the 1980s. Among these were the first domestic communications satellites and portable ground stations. They also had a switched broadband system in many cities. This was an analog dial-up network with the bandwidth of a television channel. Unfortunately there wasn’t much need for it at the time. The company way hampered by government regulations and lack of capital.

    1. In the mid-1990’s we were looking for a high bandwidth means of getting radar data from a site out in the country to our weather center. The local phone service only offered us dial up service. Then we heard that Western Union had a cable with lots of capacity running along a railroad track a half mile away. We thought our problem was solved! But Western Union didn’t want to do it…. We ended up stripping a lot of data off and sending the rest through 4 9600 baud modems through the local phone service.

  5. This is very reminiscent of a similar but different kind of paper and printer that was sold (without much success) by IBM in the 80s. I think the printer was called the 4250, and it used paper which had two additional layers: black (ink? carbon coating? not sure…) and a silver (aluminum, I think), conducting layer. The printer worked (similar to the Teledeltos) by running a current through the silver layer and eroding (evaporating?) the conducting layer to expose the black behind it. It was quite high resolution for its time — at least 400 DPI; it was intended to produce camera-ready copy. If I’m recalling this accurately, it took advantage of the fact that the current density just below the electrode was very high, but the conduction to a current sink through the coating was very low density. I never heard the name Teledeltos; I was told at the time that the paper was used for making capacitors.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.