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The Branly Coherer

August 25, 2014

Useful discoveries are sometimes practiced for many years without being understood. One example from biology would be selective breeding , practiced from prehistoric times without any knowledge of the underlying genetics involved. Our scientific understanding of the process has led to the improved art of selective breeding known as genetic engineering .

After the discovery of radio and the development of simple transmission and detection devices by Heinrich Hertz , there was a pioneering period of experimentation to develop radio into a practical method of information transmission . Generation of radio waves was easy with late nineteenth century technology ; but this was the age before vacuum tube electronics , so detection of radio waves was difficult.

Modern schematic of the Heinrich Hertz 1887 radio experiment.

Modern schematic of the Heinrich Hertz 1887 radio experiment.

(Illustration by the author, rendered with Inkscape , via Wikimedia Commons .)

The radio apparatus used by Hertz is pictured above. The radio frequency generator was a spark gap transmitter . As shown in the schematic , the core of a transformer acts also as part of a buzzer whose contacts chop a battery voltage into an alternating current excitation of the transformer primary . The capacitor ensures that the buzzer would start when the switch is thrown, it prevents spark erosion of the buzzer contacts, and it also teams with the inductance of the transformer coil to make a resonant circuit .

The transformer produced a large voltage at the secondary, which was discharged at the ball electrodes . A crude antenna , formed by the two paddle plates, transmitted the radio frequency component of the spark to a companion loop antenna . Electrodes at a gap in the 7.5 centimeter loop antenna produced a spark. A small gap was important for sensitivity, so the sparks were probably monitored by a microscope . The transmitter could produce copious radio frequency energy , but the detector was not very sensitive. It could receive signals across a room, but no farther.

The Hertz experiment was published in 1887, and a better detector was invented just a few years after that. This was the coherer , a radio detector that replaced the primitive spark gap. The basic coherer, which is shown in the figure, is a tube filled with metal powder (typically iron ) with electrodes at each end. It was invented by the French physicist , Édouard Branly (1844-1940), around 1890.

Diagram of a Branly coherer

The Branly Coherer .

An insulating tube is filled with metal powder , with the top and bottom electrodes connected to a dipole antenna , and the side electrodes connected through a battery to a current -measuring galvanometer .

Radio signals increase the conductance of the initially resistive metal powder. Tapping the tube restored the initial resistive state.

(Via Wikimedia Commons .)

Like most of science , Branly's idea didn't come out of the blue . This electrical effect of metal powder was first demonstrated around 1885, but in a non-radio electrical circuit. Branly reasoned, of course, that the source of voltage is immaterial. The physical mechanism is the dielectric breakdown of the thin, generally insulating, oxide layer that's present at the surface of most metals.

The coherer got its name, from Oliver Lodge nonetheless, from a misunderstanding of its operation. Our modern understanding of dielectric breakdown of materials was not known at the time, so it was thought that electromagnetic forces produced by the radio waves brought the metal particles closer together; that is, they cohered . This coherence was thought to decrease the resistivity.

Coherer resistance decreases with each received radio frequency pulse, and a point was reached when further signals could not be detected. At that point, an operator would tap the coherer to again loosen the metal particles into their high resistance state. After a while, this tapping action was automated (see figure). The coherer was used as a radio detector until it was replaced with crystal detectors starting in 1907.

A tapping coherer

A 1917 illustration of a tapping coherer from The Electrical Experimenter .

The electromagnetic actuator for the tapper was wired in series with the coherer, so there would be a tap after each received signal.

(Via Wikimedia Commons .)

In his initial publication ,[1] Branly reported impressive resistance changes of his coherer to radio emissions from a nearby transmitter. He reported changes in resistance from an initial state to the state after receipt of a radio pulse of several million ohms to two thousand ohms (kohm); several million ohms to 100 ohms; 150 kohm to 500 ohms; and 50 ohms to 35 ohms. The enhanced conductivity was found to persist for at least a day.[1] Branly experimented with other material systems, also, but the granular metal coherer proved to be the best radio frequency detector.[2-3]

There's still interest in the operating principle of the Branly coherer even after more than a hundred years. Granular materials have recently become a popular area of study, and Branly's coherer is a model granular system. There have been several articles on the Branly coherer posted on arXiv ,[4-8] the most recent of these, by Charles Hirlimann of the Institut de Physique et Chimie des Matériel de Strasbourg (Strasbourg, France ), was published at the end of last year.[4]

Hirlimann provides evidence that the Branly effect is caused by an induced tunneling of electrons through the oxide layer.[9] He presents a nice summary of research on the Branly effect, including the 1898 observation by Auerbach that acoustic excitation at audio frequencies will also cause a Branly coherer to conduct.[10] Other work, using a one-dimensional array of metal spheres as a model system, has demonstrated that once conduction occurs, even voltages as small as 0.4 volts will cause local heating of the contact areas to temperatures of up to 1050°C to microweld the connections (see figure).[6]

Branly experiment with metal beads in one dimension

Branly experiment with metal beads in one dimension. (Via arXiv .[6])

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