There were many attempts in the 19th and the beginning of the 20th century—like those of J. J. Thomson in 1881, Oliver Heaviside in 1889, and George Frederick Charles Searle in 1897, Wilhelm Wien in 1900, Max Abraham in 1902, and Hendrik Antoon Lorentz in 1904—to understand how the mass of a charged object depends on the electrostatic field. This concept was called electromagnetic mass, and was considered as being dependent on velocity and direction as well. Lorentz in 1904 gave the following expressions for longitudinal and transverse electromagnetic mass: