Text: [CRYSTAL] "When an alternating voltage is applied to the combination, it tends to make current flow from the catwhisker to the crystal during the first half cycle. The resistance which the crystal and contact offer to current flow in this direction is low, so the current is able to flow through easily. On the next half cycle, the voltage is reversed and tends to send current in the opposite direction. The detector contact offers a very high resistance to current flow in this direction, so very little current is allowed through. The action is repeated for each cycle. It is the difference in strength of these two currents which determine how well or how poorly the crystal operates a detector. A good crystal will almost entirely eliminate the flow of current in one direction."

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Source: 16