Kinds of Black Holes
Electrically Charged Black Holes
Imagine a non-rotating black hole that has no electrical charge. Now suppose you throw in a few electrons. As the hole begins to get an electrical charge, it forms a second event horizon. Since the black hole now has two properties, it now has two event horizons. Now, there are two places where time appears to stop.
As more and more electric charge is thrown into the black hole, the inner event horizon starts to get larger, while the outer horizon starts to shrink. The maximum possible charge on the black hole is when the two horizons come together and merge. If you tried to force in more charge, both event horizons would disappear, leaving a naked singularity.
These properties were first discovered between 1916 and 1918 by the German H. Reissner and the Danish G. Nordström. This is why electrically charged black holes are sometimes called "Reissner-Nordström" black holes.