Monthly Archives: January 2012

Anniversary of Vail’s First Demonstration of the Telegraph

Over at “This Day in History,” the lead story today is “Morse demonstrates telegraph.” It leads off, “On this day in 1838, Samuel Morse’s telegraph system is demonstrated for the first time at the Speedwell Iron Works in Morristown, New Jersey.” Well, I thought, surely this is worthy of a blog post, so I poked around on the web to learn more.

It turns out that this first demonstration was put on by Morse’s partner, Alfred Vail. Vail had first become involved three months earlier when visiting his alma mater, the University of the City of New York. He stumbled upon Samuel Morse demonstrating his “electro-magnetic telegraph” with over one-third of a mile of wire coiled around a room. Vail was hooked. He convinced his brother George and father Stephen to support further development of the telegraph at the Speedwell Iron Works, and he signed an agreement with Morse to turn Morse’s crude prototype into a market-ready model — at his own expense — by January 1, 1838, in return for a minor share.

The challenge was to get the thing to work with a length of wire much longer than Morse had managed to use. Alfred Vail recruited an apprentice at Speedwell, William Baxter, and got to work. After many frustrations, they finally succeeded in getting their model to work:

At last on January 6, 1838, the machine was ready to be demonstrated. The cotton-covered hat wire was coiled around the room on nails to equal a distance of two miles. Alfred sent Baxter to “invite Father to come down and see the ‘Telegraph’ machine work,” which sent the eager lad plunging into the cold afternoon without stopping to throw a coat over his shop clothes.

The machine that sent Stephen’s message, “A patient waiter is no loser,” was still far from perfect. A few days later [January 11] several hundred men and women crowded into Speedwell to witness the first public demonstration. The message this time had a practical cast: “Railroad cars just arrived, 345 passengers.”

http://www.morrisparks.net/speedwell/tel/tel.html

I’m not sure how these messages were formatted, but most likely they were not sent letter-by-letter. In those early days messages were laboriously sent using numbers that were assigned to commonly-used words. Eventually the “Morse Code” alphabet would replace this system, though great debate rages over who invented it.

Vail himself gives credit to Morse for the alphabetical system on p. 30 of his book, The American electro magnetic telegraph: with the reports of Congress, and a description of all telegraphs known, employing electricity or galvanism (available online for free), though some doubt the truth of this statement. Many years after his death, someone even sneaked in and engraved on Vail’s tombstone, “INVENTOR OF THE TELEGRAPHIC DOT AND DASH ALPHABET.”

Whoever invented the alphabet, what does seem clear is that Vail was the one who invented the straight key, an elegant improvement upon the cumbersome machines first used to encode messages. Today, you can even purchase a replica of Vail’s “spring key” from Kent Morse Keys!

Alfred Vail became increasingly frustrated by Samuel Morse’s lack of involvement in the development of the telegraph while publicly taking all the credit. Vail stuck with it for ten years before finally leaving the telegraph behind.

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Minimum-Loss Matching Pad

In my last post I promised to write about the minimum-loss matching pad that I’m using to couple my signal generator to the device I’m testing. The source impedance of the generator is 600 ohms and the output is intended to be terminated in a 600 ohm load, but the device I’m testing is only 228 ohms. The way to match this with the lowest loss is with a transformer, but it is inconvenient and unnecessary to come up with a transformer for every mismatch this piece of test-equipment will face.

Thanks to advice from the ham who is guiding me in this project, I’m using a minimum-loss matching pad, also known as an “L-pad,” to match these two impedances. (I’d tell you who this fine fellow is, but to keep you in suspense about my project I’ll wait until my final write-up. If I name him now, the cat will be out of the bag!) This quick, cheap, and easy match requires only two resistors:

To calculate the value of the resistors and to calculate the loss of the matching pad, use these formulas (A spreadsheet that uses these formulas is available through this webpage.):

In my case R1=472, R2=290, and the loss is -9.25 dB. That loss is pretty significant, but it is acceptable for this application. Remember this is a minimum-loss matching pad, not a no-loss matching pad. Using what resistors I had on hand to come as close as I could to the required values, I soldered this pad on a generic PC board from Radio Shack that I cut in half using my Dremel tool with a cutting wheel:


For more on this topic, I commend to you this webpage on “Impedance and Impedance Matching.”

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