A metal ring is placed in a magnetic field , with its plane  perpendicular to the field. If the magnitude of the field begins to change, the ring will experience

when magnetic field changes magnetic flux also changes an induced current develops in the ring and each portion of  the ring experience a force il x B which is perpendicular to both l and B hence it will be radially inward or outward as a result ring will try to contract or expand  hence a tension develops in the string.