He employed an instrument of his own design commonly referred to as the Eötvös balance to make extensive measurements, ultimately demonstrating to a much higher degree of accuracy than had been ever achieved before that gravitational mass and inertial mass are equivalent. Eötvös’ findings served as part of the basis of Albert Einstein’s general theory of relativity, in which the principle of equivalence acts as the keystone.
Eötvös was born on July 7, 1848, in what is now known as Budapest, Hungary. His father, with whom he had a close relationship, was Baron Jozsef Eötvös, a prominent poet, writer and statesman who advocated Hungarian political and social reform. Originally the younger Eötvös studied law in college, as was common among aristocratic families, and even obtained a degree in the field. However, his personal inclination was toward the natural sciences. In a letter to his father sent in 1867, Eötvös explained:
"I was born with ambition and a sense of duty not only to one nation but towards the whole of humanity. In order to satisfy these urges and to retain my own individual independence, my aim in life will be best achieved, as far as I can see at present, if I follow a career in science."
In 1867, he obtained his father’s permission to go abroad in pursuit of the type of scientific education he desired. He traveled to Heidelberg, Germany, where he had the opportunity to learn from such influential scientists as Robert Bunsen, Gustav Kirchhoff and Hermann von Helmholtz. He also spent a short time studying in Königsberg, but preferred the lectures at Heidelberg. When he returned to Hungary in 1871, Eötvös was awarded a doctorate in physics. His thesis concerned the work of Armand Fizeau and the relative motion of a source of light. Soon after Eötvös’ homecoming, his revered father passed away, but not before encouraging his son to continue devoting himself to science.
Abiding by his father’s wishes, Eötvös signed on as lecturer in theoretical physics at Pest University, an institution that has borne his name since 1950. Having lectured there for less than a year, he was awarded the chair of theoretical physics by the king. In 1878, he became a professor of experimental physics. He was later placed in charge of the effort to merge the theoretical and experimental physics departments into a single physical institute, of which he was appointed director. Eötvös became a member of the Hungarian Academy of Sciences in 1873, and acted as its president from 1889 to 1905. From 1894 to 1895, he served as minister of education, a post that his father had held several times before him. While working in this governmental capacity, Eötvös established Jozsef Eötvös College, a scientific institution named in his father’s honor that did not require tuition or fees from its poorest students. Always an advocate for advancing the natural sciences in Hungary, Eötvös was the first president of the Mathematical and Physical Society formed in 1891.
Eötvös’ earliest notable contributions to science focused upon the interaction between the surface of a liquid and a solid, or capillarity. To carry out his investigations, he developed a highly sensitive reflection method for observing changes in the geometry of a liquid’s surface and determining surface tension. With this method he successfully ascertained the surface tensions of a variety of liquids, which led to his realization that surface tension is related to the molar weight of a liquid. He also derived in 1886 what is commonly known as the Eötvös law, which defines the relationship between temperature and the surface tension of a liquid.
Following his work on capillarity, Eötvös embarked upon the study of a topic that would hold his attention for much of the rest of his life: gravity. As in his earlier work with capillarity, Eötvös decided that a more accurate means of studying the gravitational field of Earth was necessary, and thus invented his own technique employing a torsion balance he developed. The Eötvös balance was an extremely sensitive instrument and the first that could be used for gravitational gradiometry, but its inventor described it modestly:
"The means I use is a simple, straight stick with masses attached to each end and encased in metal, so that it will not be disturbed by the movement of air or differences in temperature. All mass near or far has an attracting influence on the stick, but the fibre, from which it is hung, resists this effect and twists in the opposite direction, producing by its twisting the exact measurements of the forces imposed upon it. This is nothing but an adapted version of the Coulomb instrument. It is as simple as Hamlet's flute, if you know how to play it. Just as the musician can coax entrancing melodies from his instrument, so the physicist, with equal delight, can measure the finest variation in gravity. In this way we can examine the Earth's crust at depths that the eye cannot penetrate and the rig cannot reach."
The initial device was constructed in 1890 and a modified model was built in 1898, the latter of which won an award at the world exhibition in Paris, France in 1900. With his torsion balance and other instruments Eötvös carried out pioneering field experiments on gravitation. Most importantly, he dynamically determined the gravitation constant and greatly improved upon prior determinations of the proportionality of inertial and gravitational mass. Eötvös received international attention for this work and a paper reporting it received the Benecke award at Göttingen University. Einstein took particular notice of the achievement, referring to Eötvös’ experimentation in his paper "The Foundation of the General Theory of Relativity," published in 1916. A few years later Einstein would further demonstrate his great respect for Eötvös by requesting his input on whom he considered the best candidate to fulfill the post of director of the Potsdam Institute of Geodesy. The field (the measurement and representation of the Earth, its gravitational field and geodynamic phenomena in three-dimensional, time-varying space), was one in which Eötvös was especially distinguished.
Over the course of his career, Eötvös developed a number of other highly precise measurement instruments, though none as famous as his torsion balance. He utilized, for instance, several different extremely sensitive implements to obtain magnetic measurements during his gravity surveys, and these were so well made that they could even determine paleomagnetic and archeomagnetic effects, a subject that he sometimes reported upon. Another device designed by Eötvös allowed him to clearly demonstrate the changes in weight experienced by moving bodies, a phenomenon commonly referred to as the Eötvös effect. As determined by Eötvös, both the direction and the speed at which a body moves across the Earth affects its weight, a fact that must be compensated for (via Eötvös correction) with gravity meters at use along the sea or in the air.
The last few years of his life Eötvös suffered from a severe illness, but prior to that time he had always been a very physically, as well as mentally, active person. He was an avid photographer and mountaineer, who often climbed peaks with his two daughters, Ilona and Rolanda, whom were born to him by his wife, Gizella Horváth. Eötvös sometimes lightheartedly claimed that his success in mountain climbing was more important to him than the torsion balance for which he had received so much acclaim. He must have been quite pleased, therefore, when in 1902 a peak in the Italian Dolomites was named in his honor. Eötvös also maintained a lifelong respect for poetry instilled in him by his father, once noting that, “Poets can penetrate deeper into the realm of secrets than scientists.” Eötvös died April 8, 1919, but not before he managed to send off one last paper for publication from his sickbed only days before he passed away, his final contribution toward penetrating that mysterious realm.