Alternator, Arc and Spark

the origins of wireless.

by Dr. Duncan Cadd, G0UTY (ex-ON9CHU)

Ahhhh! Happiness is a selective-fire FN FAL and no licence required! Oh joy!

I've always had a soft spot for technical stuff, things scientific, electrical and mechanical. I suppose that's why I ended up getting a PhD in chemistry. My interests in wireless go back quite some time. As soon as I could read, I would thumb through my Father's technical books just for the sake of having something to read and something must've stuck. Terman, Henney, Langford-Smith, Zepler and a good many others were childhood friends. I did not, of course, understand what I was looking at all those years ago, in many instances I still don't, but a fascination was born and in more recent years I have had the opportunity to build reproductions of some early wireless apparatus. First amongst these was a spark transmitter and an induction coil. I was largely inspired in this by Peter Jensen, VK2AQJ / G4GZT, whose amazing efforts using for the most part nothing more sophisticated than an electric drill spurred me into activity.

Whilst still in the construction phase it became apparent that the spark transmitter would have to be tested. It is all very well to produce something which looks the part, but unless it can be fired up and made to work it may as well be a hatstand or magazine rack as a piece of old wireless technology. I did, briefly, consider dumping the output into an inductively-coupled load resistor, oil cooled and of generous proportions, but this seemed unnecessarily boring and moreover there would be not a lot to prove that it was radio frequency currents which were producing the heat. As a child I had been given an American magazine (the 1961 Popular Electronics Electronic Experimenter's Handbook) by Dad, who at that time worked in the USA and it was with the Tesla coil project on page 15 that I first gained an experience of spark transmitters. I had commenced my adult spark transmitter project having a typically-academic view of Tesla coils and their builders ('load of childish nonsense, pah!') but as I progressed it suddenly dawned upon me that a Tesla coil would make the ideal dummy load for my spark transmitter. All of a sudden, Tesla coils became A Jolly Good Thing, and after all, repentance is the key to new beginnings! Accordingly, I wound a small tube with some spare copper wire out of the junkbox, and the initial tests of the spark transmitter led very easily and with an addictiveness to rival diacetylmorphine to the seemingly-endless cycles of tweaking and improvement to the circuitry, to the coil and hopefully to my understanding.

All manner of near-superstition attaches to Mr. Tesla and his coil. However, a decent shelf of radio books, some of them dealing with spark wireless, proved a major asset in disentangling some of the fug which surrounds these things. Ultimately, the resource left to me by my Father, most of it 1930s and 1940s vintage, allowed me finally to set straight in my own mind just how the spark transmitter-driven Tesla coil operates. This webpage exists primarily for my own good, for as one of the quotations prefaced to an article here proclaims, no invention is properly understood unless and until you can explain it to the first person you meet on the street. By attempting to explain it to you, what I am really about is convincing myself that I actually know what I'm doing!

These pages are presently a little short of photographs, though they will follow. Those which do appear are frequently from old textbooks and have an appropriate "period" quality. Please excuse me if on occasion I have chosen to include photos which are of poorer quality than might appear in publications today.

There are a fair few equations in some of the articles here. Unfortunately, the only way I have been able to include them is as .gif images. This means that if you wish to save any of these pages to your own system (which you are most welcome to do) you will need to right-click-and-save-as (or Mac equivalent) on each and every equation. My apologies for this monstrous pain, but I can see no way around it.

Articles available:

Alternator, Arc and Spark. The First Wireless Transmitters. Some of the early experimenters and some of their inventions. This is also the origin of my personal fascination with old wireless technology.

Spark Transmitter Basics. Wavelength, Amplitude, Decrement and Damping.

The Quenched Spark Gap. A reasonably authentic working reproduction.

Maximising Power 1. The maximum power theorem. Resistance, reactance, impedance and resonance. Tuned circuits. Wattless power vs. in phase power.

Maximising Power 2. The meaning of Q. Tank and link circuit design. Skin and proximity effects. Litz wire and ferrite cores.

Conductors at Radio Frequency. Tube or flat strip? Why a copper strip 0,206mm thick has 8,3% less resistance at 1Mc/s than any other thickness.

Inductive coupling of tuned circuits. The coupling constant k. The effect on Q and decrement of varying the value of k. Maximising the output by slight detuning.

Tesla's Magnifier and Transmission Lines. Problems. A solution. The low impedance magnifier: 50 ohm tuned link coupling.

So what exactly is a Tesla Coil? Electrons in a bottle. Spot the difference! AC vs DC charging of isolated electrodes. Polarisation effects and antique wireless receiver technology. Maximising the charge in the bottle. Intermediate Frequency Transformers.

The Induction Coil Another reconstruction project, I've wanted to build one of these for years, and Peter Jensen's book finally got my backside into gear! This little coil is used to generate a few watts of rf to drive a miniature Tesla coil. New addition to this site September 01.

Other Stuff:

The Synchronous Rotary Mechanical Bridge Rectifier. How to rectify 250kV without diodes and generate 0,05 Angstrom x-rays.

The Quick & Dirty VTTC. A very simple, low power valve-driven Tesla coil. It generates a small but intensely hot plasma "flame". Now (September 01) updated with more photos.

Links Page Favourite links on a variety of subjects.



My email is currently (24/12/06) disabled due to over 300 spams a week. Hopefully it will be back in early 2007.