Coherer
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The coherer (from Latin cohaerere "to connect ", English coherer ), in German also known as fritter (from the verb fritten for "to bake together"), is used to detect electromagnetic waves in the frequency range of radio waves . This makes it a predecessor to the detector diode . In the original design, it consists of an electrically insulating tube, usually glass, but also hard rubber / ebonite , which is partially filled with metal shavings , for example iron filings ("iron filament"). The tube is terminated at both ends with electrodes through which the electromagnetic waves to be displayed are fed from the antenna .
The coherer was developed in the 1880s by the Italian Temistocle Calzecchi-Onesti . However, his reports in the Italian journal “Il Nuovo Cimento” received little attention from experts outside of Italy. Separately, in the 1890s, Édouard Branly developed a relatively sensitive indicator for electromagnetic waves. Coherers are no longer used today and can be described as historical devices.
function
In the basic state, the section between the two electrodes, which is partially filled with metal chips, is practically electrically non- conductive . If the electromagnetic waves captured by the antenna hit the coherer, the connection becomes low-resistance as a result of exceeding the fritting voltage , also known as the fritting voltage .
Since the coherer remains conductive after the arrival of electromagnetic waves, it must be returned to the non-conductive state. To do this, it is sufficient to shake it mechanically, which z. B. in the telegraph is effected by a clapper, similar to the clapper of an electric bell. This clapper, also known as a decoher , is controlled by the telegraph relay at the same time as the Morse code writer and functionally corresponds to Wagner's hammer . It is important that the clapper is mechanically arranged in the direction that when the signal is interrupted, the clapper is hit on the fritter by spring force and thus makes the fritter non-conductive. Only when a new signal arrives will the clapper and possibly the relay move again.
The combination of fritter, clapper, telegraph relay and Morse code writer made it possible in the early days of radio technology to automatically record what were then known as " wireless telegrams ".
Mercury based coherers
The mercury-based coherer was an extension that avoided the mostly complex mechanical constructions for resetting to the non-conductive state. It consists of a flat bowl filled with mercury . The mercury acts as an electrode. Above this there is a thin layer of electrically non-conductive oil. A thin metal plate is dipped into the oil layer from above as a second electrical contact. The metal plate is adjusted in such a way that the thin oil film between the mercury and the metal plate does not tear when it is idle - these two connections are thus electrically isolated from each other. If a high-frequency signal with sufficient voltage hits the two electrodes, an electrically conductive connection is created between the connections, which is also permeable to direct voltage. The connection breaks automatically when the high-frequency voltage is switched off.
The disadvantage of this construction is the necessary precise adjustment of the metal plate and the position sensitivity of the arrangement, which is why the mercury-based coherer was not widely used. The mercury coherer was very delicate. It was invented by Luigi Solari after a discovery by Prof. Tommassina in Geneva in 1899.
Anti-coherers
“The anti-coherer consists of a thin metal foil that is cut through by a zigzag-shaped fine line so that this dividing line is as long as possible over a small area. If you connect the two film halves, resp. their connection terminals in such a way that an element , a vertical galvanometer and the anti-coherent are connected in series in a circuit, the galvanometer will show a deflection if a slight layer of moisture is created on the metal plate by gently breathing in. If electrical waves hit the device, the resistance of the anti-coherent suddenly increases and the galvanometer returns to its zero position, only to give a deflection after the irradiation has ceased . ”The anti-coherer is also known as an anti-knight .
Web links
Radio technology ( Memento from April 30, 2009 in the Internet Archive )
Individual evidence
- ↑ What does fritten mean? Retrieved August 18, 2016
- ↑ MARCONI 1942, Luigi Solari, Paul List Verlag Leipzig, page 43
- ↑ pricelist PHYSICS 1905, Messrs. Max Kohl Precision mechanics in Chemnitz, Chemnitz in Saxony