anorlunda said:
THANK YOU
@tech99 . It is not every day that I get to add an electrical word to my vocabulary. I never heard the word coherer before.
That also sounds like a fun project. I bet you could adjust the gap to trigger at different voltages.
The threshold voltage seems to depend on the materials and is just a few volts. There is also a "linear" mode of operation allowing detection of signals of around 50mV without amplification. The following notes might be of interest:-
The device was used as an early detector of radio waves. It is usually described as a glass tube in which two metal electrodes are placed, the gap between them containing some loose metal powder or sometimes a drop of mercury. There are other types known, some of which resemble semiconductor diodes, but which might or might not employ semiconductor action, and others which are just light contacts.
A bias battery of typically 1 volt is connected, no current flowing under rest conditions, but when the voltage is increased by the addition of RF voltage to the battery potential, a direct current starts to flow. This can create a click in a pair of earphones, and if the signal is amplitude modulated, the modulation can be heard. If the RF voltage is fairly large, say 1 volt or more, and the resistance in the circuit is small, the contact can micro-weld itself closed, so that a large current can flow from the battery. This produces a latching action, able to operate a relay, and a mechanical reset was often used in the form of an electromagnet to shake the coherer and break the contact.
The operation of the coherer is seen to have two modes: a “linear” detection mode and a latching mode. It does not appear to involve the cohering of the metal particles together, and such a mechanical action seems unlikely in view of the use of the device at 60 GHz by J C Bose in 1895.
The linear mode seems to occur due to an oxide film on a contact surface, and the response seems to be an S-shaped curve which is symmetrical with respect to battery polarity. By biasing the device a little way up the curve, an asymmetrical action occurs, and when AC is added to the bias potential, rectification occurs. However, this does not appear to be semiconductor rectifying action, as it occurs with either battery polarity, even though the materials used might resemble those of a semiconductor diode or crystal detector. The action has been said to resemble that of a Metal-Insulator-Metal (MIM) diode, where tunnelling occurs through an insulating oxide layer. My own observations support this, as I found the action to be nearly always symmetrical, and it did not occur with carbon, which does not form an oxide film. A particularly sensitive design uses copper electrodes with a drop of mercury between them, and it seems likely that the oxide barrier consists of copper oxide. It also works with iron, zinc brass etc but not carbon.
There is no need for the signal to be AC; the coherer is just responding to an increase in voltage, either as a non linear conductor or a threshold device, and frequency has no relevance.
Maybe the first demonstration of radio communication took place in London in February 1880, when Professor Hughes obtained a range of 500m using a mobile receiver having a steel/carbon contact. The mercury coherer was used by Marconi in 1901 for the transatlantic test, in conjunction with a very sensitive earpiece. In my own tests I was able to hear HF broadcasting with a coherer and obtained sensitivity approximately 10 dB inferior to that of a Germanium diode.
