[Paraphrased from the excellent rec.audio.pro FAQ at
http://www.cis.ohio-state.edu/hypertext/faq/usenet/AudioFAQ/pro-audio-faq/faq.h
tml
]

Condenser microphones have internal electronics which require power. In
phantom power (DIN spec 45596) the positive terminal of a 48V power
supply is connected via 6800ohm resistors to both signal leads of a
microphone and the negative terminal to the ground connection

A dynamic or ribbon mic can be connected to a phantom powered circuit
without damage. The only risks are a shorted mic cable, or some old mics
with a centre tap - these will be damaged if connected to a phantom
powered circuit.

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