An elemental sphere comes in different sizes, with the damage it wreaks corresponding with how large it is. Trapped inside are all of the four elements (air, water, fire, and earth) together. When released, the elements escape and hit anything near the sphere. It is usable even if one is not a magician, so long as the sphere is small; larger ones generally have to be broken in some kind of magical means.
In the books[]
Martin's sphere
They are first seen when the government is attacked in The Amulet of Samarkand by Martin, a member of the Resistance. The one used was a small, relatively low-power sphere, although it is hinted that the effects were amplified by the size of the room. It did not affect many major magicians, but a great many minor magicians and their guests suffered various minor injuries and indignities due to the orb.
They are also mentioned or seen a few times in the parts of the book from Kitty's point of view, having either been stolen by the Resistance or used by them to aid in their crimes.