Einstein’s life-long effort to develop a theory that unifies gravitation and electromagnetism was not a purely theoretical enterprise. The technical environment of a gyrocompass factory triggered his search for a novel connection between the rotation of an electrically uncharged body and its magnetic field. The dimensional equality of the electric unit charge and the mass of a body multiplied by the square root of the gravitational constant hinted at a nonsensical electric charge, to which he gave the name “ghost charge.” He felt that he found a fundamental unity of gravitating mass and electricity, a hitherto undiscovered law of nature. Two physicists offered to assist him in finding evidence of this peculiar electric charge. Peter Pringsheim performed experiments with deionized gases and Teodor Schlomka made measurements of the earth’s magnetic field from balloons and airplanes; Schlomka also executed a thorough literature search and placed Einstein’s efforts in their historical context.