Diodes allow current flow when forward biased and stop current flow when reverse biased.? However, there is always a voltage drop when forward biased caused by the potential difference at the p/n junction.? That difference must be overcome to allow correct to flow.? Fo silicone diodes the difference in potential is approximately 0.7V.? If you measure the voltage across a diode not connected to a circuit you'll see that voltage in the forward direction.? That causes you voltage reading of the battery to be about 0.7V less when measured through the diode than when measured across the terminal that do not have a diode present.

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