Monday, January 7, 2019

The Coherer

Have you ever heard of a device called the Coherer?  The Coherer is an early radio signal detector that was used over 100 years ago. It evolved as a replacement for the Hertz detector [copper loop conductor with an air gap].   We know that Hertz was the first to detect electromagnetic waves in 1888 proving Maxwell’s earlier mathematical predictions. The Coherer, a primitive form of radio receiver, was a significant participant in the very early days of wireless.

Branly in his Lab
Professor Eduard Branly discovered the Coherer around 1890. His device transformed the development of wireless technology as Marconi used it in his first wireless transmission system later in 1896. 

Branly was a French Physicist that resumed research on electromagnetic conductors began by others earlier in the 1880s. His research centered on iron filings [in powder form]; and their reaction to electromagnetic waves. The Coherer is essentially a small glass tube with two pieces of metal with wires attached to each end thus forming electrodes.  The two metal pieces are placed in proximity to each other to form an air gap where iron or copper filings were positioned.
Branly's Coherer

In his early experiments, Branly found that the metal filings, while lying on a glass plate, changed their resistance when in proximity to a strong electromagnetic wave [signal oscillations]. He explained this phenomenon by stating that the filing’s “cohered” [stick together, hold together].   Branly was also a medical scientist who researched different theories about how nerves carry messages from the skin to and from the brain.  His research found that nerves are not continuous fibers but neurons. Neurons are a bunch of cells gathered closely but are not connected. He equated this neuron phenomenon to describe the coherer effect.

Branly encapsulated the filings in a glass tube.  He attached a battery circuit and a galvanometer [type of an amp meter] to the coherer; then discharged a Leyden jar whose electromagnetic wave [oscillations] made the loose metal filings cohere; that is, form a low resistance circuit, moving and sticking the filings together to act like a conductor. This completed the circuit allowing the current to flow [as the galvanometer would indicate]. Disrupting the filings [separating the loose filings] would stop current flow until another Leyden jar discharged was conducted for them to cohere again.

This device conceptually opened the door to wireless transmission as Branly showed that it could detect electromagnetic waves. The coherer is considered a key element in the development of wireless, and earned Branly a Noble Prize in 1921. This technology became the standard for early wireless telegraphy receivers until the “cat whisker” detector was discovered to be a better detector in 1906.
Facsimile of Lodge's coherer circuit
Circuit Digram
In the mid 1890s, the English Physicist Olive Lodge experimented with Branly’s coherer. Lodge found that it was superior to his detector device and experimented with it for his wireless transmission system. Lodge was a noteworthy wireless scientist who conducted many important wireless experiments prior to Marconi arriving on the scene.  Lodge improved Branly’s device by adding a “tapper” that shook and loosened the filings after they cohered in between transmissions [clear the filings when the oscillations ceased].  The Lodge coherer circuit included a bell that would ring when the filings cohered; stop ringing when the electromagnetic wave dissipated, then the tapper cleared [shook] the filings apart for the next transmitting/receiving sequence.  Lodge is also responsible for naming the device the “Coherer”.

Gugliemo Marconi, experimenting with his wireless system, used the Branly’s coherer and the tapper that Lodge developed to transmit and receive wireless Morse code. He also improved the device by adding vacuum chamber to the coherer, thus making it easier for the filings to cohere. Marconi first successful radiotelegraphy transmission and reception occurred in 1896 [about 40 miles], which resulted in his first radio Patent.  He was also responsible for the first transatlantic radio transmission in 1906.
Marconi's Coherer