Angle formed by the direction of the terrestrial magnetic field with the horizontal plane p in a given point of the Earth's surface P. The dip is called positive if the north pole of the magnetic needle faces downward, negative if it faces upward. The magnetic dip is equal to 0° at the equator and increases—positively in the northern hemisphere, negatively in the southern hemisphere—as one approaches the magnetic poles, where it is equal to 90°. In Italy, its value is approximately +57°. The lines joining points of the Earth's surface with equal dip are called isoclines. In any given point, magnetic dip experiences a slow variation over the centuries, a 24-hour change, and irregular fluctuations due to magnetic storms. The magnetic dip is measured with instruments called inclinometers. These basically consist of (1) a magnetic compass whose needle rotates freely around a vertical axis on the magnetic meridian plane, and (2) a goniometer that measures the needle's dip to the horizontal. In modern times, the induction inclinometer (also called dip inductor or terrestrial-induction compass) is preferred for its greater speed and accuracy. It consists of a coil rotating around an axis that is moved until the coil has lost all of the current induced by the terrestrial magnetic field. The coil axis is then aligned with the direction of the magnetic field.