A coil has multiple coil sections connected to each other and each coil section includes a body portion and at least one direct or protrusive connecting portion disposed at one end of the body portion. Coil sections form at least one spiral path around the central axis of the coil, and on the projection of the coil along the central axis. The protrusive connecting portions protrude out of the path location of the direct connecting portions. Two connected coil sections form only one overlapped surface at the coupled parts of the direct or protrusive connecting portions. Regarding to the body portions in the same spiral path, a first end of one body portion is indirectly connected and disposed adjacent to a second end of another body portion. The second end has one surface with a virtual extension reaching the first end.