ABC Logo
The Basics of Tesla Engineering Principles
Coherers, Tubes and Diodes...
Steven R. Elswick, BSEE -!- Publisher/Editor of ExtraOrdinary Technology

Reprinted from: ExtraOrdinary Technology (Volume 1, Issue 2; Apr/May/Jun 2003)

Back to Basics...
We live in an exciting age where anything that is conceivable is possible. Perhaps the most important segment in this magazine was the “proof” that Tesla did have a car powered by cosmic rays. In going through the “proof” and developing the arguments pro and con, I must admit I went from skeptic to true believer. Now, firmly convinced it did exist, we can move into the next phase of actually recreating the device, or developing a derivative.

To do so we must have a true working knowledge of Tesla’s applicable patents, the various components and circuits used in such devices, and information on similar devices that may have worked. For this, we must have a basic understanding of what each component in the circuit did... many which are no longer in use today. In short, we need to be able to read Tesla’s material in the same context that Tesla wrote it in.

Quite frankly, an entire book can ... and has... been written on each of the various components. With that in mind, in this column, we will be running a series of articles covering the basics and reviewing some “obsolete” methods of signal processing technologies, including power circuits. This is intended to be a brief overview, so often, we will include a recommended reading list for additional more-in-depth material.

Our primary focus is on receivers, not transmitters. Cosmic rays, our ultimate target, are naturally generated. The receiver is what we use to capture them and convert them into useful power.

Hertz Resonator
Developed and used by Heinrich Hertz in his famous “radio” experiment, the first and simplest radio wave detector was the Hertz resonator. It consisted of an open metal ring with a small metal sphere attached to both ends. Whenever a spark was generated in the transmitter, a spark jumped across the small gap between the spheres.

Ultimately, the Hertz resonator was a “proof of concept” device. It was quickly replaced with an improved detector... the coherer.

You most likely will not find this term in any modern circuit textbook today. I went through four years of standard electrical engineering courses, including both graduate and undergraduate level classes, and was stumped the first time I came across this term reading Nikola Tesla’s Colorado Springs Notes!

A coherer makes use of what today would be called a “dry joint” which is formed by an imperfect solder connection. This type of contact between two conductors results in a large resistance between the two. However, the application of AC or DC voltages (starting from a few tenths of a Volt) can cause a large decrease in resistance!

Typical Coherer
The phenomenon of coherence, first observed in 1835, found its first application by 1852 as a lightning protector for telegraph lines. Under stress of a high voltage lightning strike, the coherer would short to ground, thus diverting destructive currents away from the rest of the system. Later, the coherer played an important role in making the development of radio possible.

In 1884, Professor Edward Branley’s studies of nerve impulses led him to develop the ‘coherer’ as a device for detecting radio signals. His device was a glass tube filled with metal filings and two electrodes. The device decreased in resistance in the presence of electrical energy, as the filings stuck together–or ‘cohered’.

By the early 1900’s Marconi had greatly improved the performance of coherers. In his design, they contained nickel filings (95%) and silver (5%). The filings lay between two silver plugs which had been amalgamated with mercury. The tube was evacuated to prevent any further oxidation.

Tesla’s Rotating Coherer...
Many coherers utilized a small hammer-like device to tap the tube after each signal, breaking up the filings and increasing the resistance in preparation for the next signal. A number of ingenious mechanisms such as shakers were developed for decoherence.

The Colorado Springs experiments led Tesla to the discovery of cosmic rays. For his Colorado Springs experiments, Tesla developed a rotating coherer. A clockwork drive mechanism was used to continuously rotate the small glass cylinders thus decohering the chips after each received impulse from lightning strikes. It is now believed by Dr. James Corum that coherers are uniquely suited to detect the particular type of natural radio phenomenon that Tesla observed in 1899.

thermionic valve
Thermionic Valve
Magnetic Detectors...
The widespread use Marconi made of the coherer exposed its poor efficiency. In response, Marconi developed the magnetic detector which employed clockwork mechanical power to magnify the weak incoming pulses from a spark transmitter so they were strong enough to drive a pair of headphones.

An intricate device, it was a significant improvement over a coherer. Patented in 1902, it was widely used for about 20 years, despite the introduction of the thermionic tube circa 1904.

Cat Whiskers...
The popular cat whisker used in crystal sets emerged at this time as well. Employing a fine wire electrode whose pointed tip was pressed against the crystal of a detector ( often galena), it was simply connected to a properly tuned circuit, antenna, ground and headphones to make a useful receiver for local stations.

Thermionic Valves...
Often referred to as a self-restoring coherer, the thermionic valve was the forerunner of the triode vacuum tubes. Conceived by J.A. Fleming in 1904, it was the first practical application of the Edison Effect (The emission of electrons by a heated element) and used as a vacuum tube rectifier and detector.

tube truth table Due to the fact that it consists of two active elements (a cathode and an anode) it was referred to as a diode. Today, the most basic semiconductor device is a diode. It also has two active elements, and performs the same operation. The difference is in construction and materials used which we will not get into here. Do not be confused by the terminology. Keep in mind the time frame when confronted with a word with dual meanings!

The early 1900s diode consisted of a glass bulb containing a carbon filament (the cathode) surrounded by a cylinder of thin metal (the anode or plate). When the cathode was heated, it would emit electrons. If the anode plate was positive (+) it would attract the electrons and a current would flow through the filament-plate circuit. If it became negative no current would flow.

Technical Evolution
of the Vacuum Tube

fleming device
Fleming Device - 1904
J.A. Fleming

Heated cathode emits electrons. Replaced coherers in radios.

Audion device
Audion (Triode) - 1906
Lee de Forest

Control grid increased stability. Replaced Fleming Valves in radios.

Tetrode device
Tetrode - 1919
Walter Schottky

Screen grid reduced capacitance. Enhanced performance at the higher frequencies coming into use.

Audion device
Pentode - 1926

Suppression grid eliminated the discontinuities in operating characteristics caused by screen grid.

Audion device
Low Power Pentode

Separating cathode from heater reduced power requirements. Enabled the use of a separate power source.

It was a major advance in signal processing and set off a race by major researchers to find a way to utilize the “Edison effect” without infringing on Fleming’s patent.

By 1906, Lee de Forest had discovered that by placing a third electrode (the control grid), one could control the flow of electrons. When the grid was negative with respect to the filament (cathode), the electron stream was repelled so that the anode current was reduced. When grid potential was positive, the anode current increased with an amplifying effect!

Initially, the triode proved to be a sensitive detector, but it wasn’t until 1912 that de Forest brought the triode or Audion) into its true glory as an amplifier. In the interim, a number of improvements were developed for optimal performance.

The first issue was the vacuum itself. At first, the small amount of gas in the Audion seemed to enhance its function. However, both the Audion and Fleming tubes suffered irregularities stemming from the gas within them. As they were used, the amount of gas within them actually increased, leading to the breakdown of the tube, due to the increased bombardment of positive ions upon the filament.

It was discovered by Langmuir that the reduced vacuum in the tube was caused by gas atoms contained in the molecules of the filament, as well as those in the metal and glass parts of the tube. In addition to constructing new pumps to create a hard vacuum in the tube, he devised new procedures to expel gas atoms from the glass and metal parts of the tube, and sealed filaments inside the bulb at high temperatures.

Meanwhile, Lee de Forest led the way to the introduction of metal filaments in place of carbon elements found in earlier versions. Efficiencies were further improved by coating filaments with alkaline oxides. The basics out of the way, further improvement in tube operation was to come directly from new design innovations concerning the very internal structure of the tube itself.

The structure of the triode (filament-grid-plate) was inherently an “active” capacitor.... all tubes are to some degree. As a result of the high grid-plate capacitance, the triodes would burst into oscillation when they were used at frequencies above a few hundred kilohertz. This form of instability is referred to as anode-grid feedback.

In 1919, it was discovered by Walter Schottky that by placing a grid between the anode plate and the control grid, the grid-plate capacitance was reduced to almost one-hundreth of that in a triode. The second grid acted as a screen to prevent the anode voltages from affecting the control grid.

However, in solving one problem another was introduced... a discontinuity in its operating characteristics caused by electrons bouncing off the anode. The solution was the addition of a third grid called the suppressor grid.

One of the main problems remaining with tubes was their inherently high power requirements. Circuit configurations were limited because the heater and cathode were one and the same. It was discovered that the cathode could be indirectly heated and this meant that the heaters could be electrically isolated from the cathode. This had the advantage that the heaters did not need to be run from a battery supply supplying DC. Instead an AC supply derived from the mains could be used. This was a major improvement because it meant that size of radios could be considerably reduced as could their running costs.

Beam Tube Structure of beam power tube
showing beam-confining action.
Beam Power and other Tubes...
This type of tube is particularly important to our study as the 70-L-7 tube is a beam tube. In the beam power tube, the basic four-element structure of the tetrode was maintained, but the grid and screen wires were carefully arranged along with a pair of auxiliary plates to create an interesting effect: focused beams or “sheets” of electrons traveling from cathode to plate. The new arrangement resulted in lower screen current (and more plate current!) than an ordinary tetrode tube, with little added expense to the construction of the tube.

The introduction of the tetrode and pentode brought revolutionary improvements in performance. As a result the use of tubes rose dramatically. In addition to radios, they were used in our earliest computers and other electronic applications. By the late 1930s many thousands of different types of tubes were being manufactured... but Bell Labs introduction of the transistor doomed the vacuum tube as semiconductors supplanted them.

Although semiconductors found widespread use in low power applications, tubes are still valuable in high power applications. Besides, to a tube aficionado, nothing can replace that warm glow and distinctive hum of a tube-powered device!__SRE
Tesla Tech Logo
TeslaTech Inc.
296 East Donna Drive, Queen Valley, AZ 85218
Order Hotline: ( 520 ) 463-1994 -:- Editor Office: ( 520 ) 463-1994

Annual Membership: $30-US  ·   $35-Canada/Mexico  ·   $55-Else


Home | Feedback | Events | Magazine | Radio | Books

Free JavaScripts provided by The JavaScript Source

Copyright © 2002-2006 TeslaTech, Inc ALL RIGHTS RESERVED