It tuning fork
It is for tuning a musical instrument. To do this the player holds it by the single tip (shown at the left hand side here) and strikes the other end against something solid. The two prongs of the fork vibrate. The tuning fork is then held with the single tip on a table or other surface which vibrates with the tuning fork and a note is heard. Most tuning forks play the A above middle C. The player can then use this note to tune their instrument.