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This paper offers the first translation into Spanish of the letter De magnete by Peter the Pilgrim of Maricourt dated in 1269. Following an experimental method, the author describes the polarity of the magnets, the laws of attraction, the immediacy of iron, the process by which polarity changes, the orientation of the lodestone to the northern and southern poles of the Earth, as well as the construction of three physical instruments with a practical use based on knowledge of the nature of the magnet: a perpetual motion wheel and two types of compasses. The article is headed by a brief introduction in which the available biographical data on the author is exposed and the work is histrorically and thematically contextualized.