When Einstein introduced Special Relativity in 1905, he began with a simple example involving a magnet and a wire coil. If you move a magnet toward a stationary wire coil, an electric current is produced. The same current is produced if the magnet stays still and the coil moves toward it. In 19th-century physics, these two situations were explained differently depending on which object was moving. Einstein noted that the physical result was the same in both cases. He concluded that the observable effects depend on the relative motion between the two objects. This became an important starting point for Special Relativity. About ten years later, while developing General Relativity, Einstein considered another experimental result: the Eötvös experiment. Hungarian physicist Loránd Eötvös used very sensitive torsion balances to test whether inertial mass and gravitational mass were exactly equal. Inertial mass describes an object's resistance to acceleration, while gravitational mass describes how strongly it responds to gravity. Eötvös compared objects made from different materials and found that the two types of mass were equal to very high precision. Einstein used this result in developing the Equivalence Principle. The principle states that the effects of acceleration and gravity are locally equivalent. This became a central part of General Relativity. In 1915, Einstein also tested General Relativity using the orbit of Mercury. Astronomers had observed that Mercury's closest point to the Sun, called its perihelion, shifted by about 43 arc-seconds per century more than Newtonian physics could explain. Einstein used his new gravitational field equations to calculate Mercury's orbit. His calculations correctly predicted the unexplained 43 arc-seconds. This provided strong evidence that General Relativity correctly described Mercury's motion. Einstein was reportedly so affected by the result that he experienced heart palpitations.