The Demon of Electricity is a powerful genie-like being, a sort of patron spirit of the natural phenomenon of electric energy.
In the first instance of its manifestation to a human being, the Demon burst forth in blinding, scintillating light. It appeared as a male figure dressed in slacks, suitjacket, and vest; though whether this is its normal state, or a guise assumed to meet the expectations of the young American male who summoned it, is uncertain. The Demon manifested a brilliant, dazzling figure, his jacket a wavering mass of white light, with golden buttons and a trim of flaming red braid that shot small tongues of fire in all directions. His slacks were an incandescent blue, and his vest blazed with all the colors of the rainbow, blending into a flashing display. The apparition had searching eyes and a low clear voice.
Though an entity of great force, the Demon also described itself as the slave of the master key of electricity: he-or-it must appear when that master key is struck, by anyone wise and brave enough (or foolhardy and lucky enough) to do so.
The Demon is capable of traversing space, and has investigated all the planets in our solar system; it knows firsthand that none of them but Earth is inhabited by intelligent beings. The Demon also can feel and express human emotions like anger, impatience, surprise — the whole range of such responses. (An irritated stamp of his foot generates thousands of sparks.) The Demon can also sit in a chair, indicating a possession (or assumption) of a physical presence.
Though called a "demon," the spirit of electricity is a strongly positive entity; it wants to bestow the gifts of applied electric energy upon human society, like Prometheus giving fire to humanity. (The Master Key)
Fanny Cory draws the Demon in some of her pictures for Baum's book — though she makes him a more exotic, robed and jeweled presence than the author describes.