Certain natural minerals can be used to detect radio signals, including zincite, silicon, germanium, galena, molybdenite, bornite, tellerium and carborundum. Rectification takes place at the point of contact with a sharp metal point of wire. A precise contact called a "catswisker" had to be made for the device to operate. This allows current to pass in one direction but not the other, and so the high frequency radio wave carrier signal is "rectified". It is converted to provide a direct current, varying audio signal that can be heard with a sensitive high impedance earphone.
The synthetic carborundum detector is a more reliable detector. Unlike the natural minerals that use a cat's whisker it is stable, but it requires a negative potential of 1 volt to be applied to it. This is to be supplied by a battery and adjusted with a potentiometer.
The potentiometer should allow about 1 milliamp to flow from the battery. Measure the current flowing in one of the arms from the potentiometer, adjust until the current just starts to flow. Alternatively, listen on the earphones to a weak station and adjust for maximum loudness.
What is not generally known is why there is a difference in performance from one crystal species to another. The fact is that a crystal mineral detector has a threshold conduction voltage. When it is forward biased it does not initially conduct until the voltage reaches a threshold value at A. There is a slight curve between A and B, then a relatively straight line from B to C and beyond. The initial threshold voltage varies from mineral to mineral.
The curves A to B and B to C are much the same for all crystal detectors. They all have right around the same frequency responses between them. Below is a partial list of crystal detectors and their respective voltage thresholds.
Moray Lead (radiant energy detector) = 0 volts?
Moray Swedish Stone = .1 volts?
Lead Sulfide (Galena) = .3 volts
Germanium = .3 volts
Silicon on metal (Schottky) = .4 volts
Silicon = .6 volts
Silicon Carbide (Carborundum) = 3 volts
DECREASING THRESHOLD VOLTAGE WITH RADIOACTIVE DOPANT
The mineral detector can be permanently forward biased so that it starts operating no matter how small the radio signals that are applied to it. This is accomplished by creating synthetic crystals that are doped with a minute amount of radioactive material. The radioactive dopant decreases the diode's electrical conductivity or resistance to electrical current. The mineral detector can be permanently forward biased so that it starts operating no matter how small the radio signal that is applied to it. This has definite advantages.
You may have heard of circuits that use multiple diodes, that increase the output of a crystal radio. This is a somewhat incorrect assumption, each detector added increases the resistance and the voltage drops. The earphones have to have a correspondingly higher resistance, an extra 2000 ohms for each detector that is added. It must also be remembered that we are limited by what the aerial and earth system can deliver to the crystal detector.
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