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the whozeejigger 4003 is a common generic diode.

Common Diodes allow current to conduct in one direction and restrict flow in the other. That is why the polarity test -- low resistance in one direction, high in the other.
So if one were to put a diode in a dc series circuit, it would conduct in one direction, but if you flipped the leads over, it would not.
Kind of a one way valve in the plumbing world..[ which is a good way to visualize / compare electon flow]

They are commonly used to rectify a/c current into DC current, but have many other applications...