In 1901 he received his doctorate with a thesis on the diurnal variation of the horizontal component of the wind, from which time began to investigate the properties of the electrolyte, the resistance of the manganin and the change in resistance of nickel and iron within a magnetic field, work whose results were published in the Annals of the Spanish Society of Physics and Chemistry.
Soon said to specialize in the study of magnetic properties of matter, a subject to which he devoted most of his life as a researcher. In 1905 obtained by opposition the chair of electricity and magnetism at the University of Madrid. Some years later, in 1910, the Board created the Advanced Studies Physical Research Laboratory, whose address was given to Cabrera. As lab director, contributed substantially to promote physics research in Spain.
In 1912 he obtained a pension of that Board to visit various laboratories in Europe and continue his research on magnetism. In May of that year came to Zurich, where he worked in the physics laboratory of the Polytechnic, directed by Peter Weiss. In this laboratory conducted research on magnetism, particularly magnetochemistry, in collaboration with Enrique Moles and Ormella.
Also visited the laboratories of physics at the Universities of Geneva and Heidelberg and the International Bureau of Weights and Measures in Paris. On his return to Spain implemented the Physical Research Laboratory in Zurich learned techniques, with different variations and improvements, and continued his work of magnetism, many of them produced in collaboration with the aforementioned Moles, Arturo Duperier and other researchers.
Between 1910 and 1934 Cabrera published around one hundred ten jobs. Contributed to the knowledge of the magnetic field of Hund and Van Ulleck, established the law that regulates the variations experienced in the periodic system of the elements the magnetic moments of the atoms of iron family ("Cabrera curve"), and gave a theoretical interpretation thereof. Further modified Curie-Weiss law for the rare earths, derived an equation for the magnetic moment of the atom including the effect of temperature and improved many experimental devices.
In addition to his important work as a researcher and as a promoter of experimental theoretical research in Spain, Cabrera also developed considerable task of introducer and promoter of modern physical theories. In 1908, at the First Congress of the Spanish Association for the Advancement of Science, Cabrera gave a lecture on "The theory of electrons and the constitution of matter". The conference explained first in Spain the Michelson-Morley experiment, did a review of "ether drag" showing its contradiction with the experiment of Bradley and explained the hypothesis of Lorentz-Fitzgerald contraction and significance of the equations Lorentz transformation.
In 1910, Cabrera entered the Academy of Sciences of Madrid giving a speech on "The ether and its relationship to the matter at rest", which analyzed the role played ether concept in physics, subjecting to criticism that notion , in 1912 published in the Journal of the Academy of Sciences of Madrid a paper entitled "Basic principles of vector analysis in three-dimensional space and the Minkowski Universe". This article Cabrera explained the foundations of the theory of special relativity.
This same year, 1912, Esteban Terradas published a comprehensive review of the book entitled Das Max von Laue Relativitätsprincip, appeared last year. With these works Terradas Cabrera and Spain were announced in the theory of special relativity. Years later, in 1923, Cabrera himself published a book entitled Principles of relativity, dedicated to special and general relativity.
Cabrera's work and other scientists, as Michelangelo Catalan and Julio Palacios, prompted the creation in 1932 of the National Institute of Physics and Chemistry, with the help of a grant from the Rockefeller Foundation, Institute later to be named director. In addition to various international conferences and scientific meetings in which he participated as a delegate from Spain. Cabrera was a member of the International Bureau of Weights and Measures (of which he was secretary general), corresponding member of the Academy of Sciences in Paris and, since 1928, was part of the Scientific Council of the International Institute of Physics Solvay (the proposal was made by Marie Curie and Albert Einstein).
In Spain he was rector of the University of Madrid, president of the Academy of Exact, Physical and Natural Sciences, member of the Spanish Society of Physics and Chemistry and a member (since 1936) of the Spanish Academy. After the Civil War, Cabrera, like many other Spanish scientists, had to exile and moved to Mexico where, from 1941 until his death he was professor of the university in the capital