The words pentacle and pentagram (a five-point unicursal star) are essentially synonymous, according to the Online Oxford English Dictionary (2007 revision), which traces the etymology through both French and Italian back to Latin, but notes that in Middle French the word "pentacle" was used to refer to any talisman.
The "electric pentacle" is a fictional electronic device invented by author William Hope Hodgson and used in several of his short stories about Carnacki the Ghost-Finder.
The Symbol itself is the equivalent of the Egyptian Ankh or the Rosicrucian's Rosy Cross; which represents the microcosmic forces (the pentacle, representation of the pentagram with 5 elements, the tetragrammaton or YHVH) interweave with the macro-cosmic forces (the hexagram, the representation of the planetary or heavenly cosmic forces, the divine).