The tuning fork was invented in 1711 by John Shore (1662--1752), a British trumpeter. Shore came from a musical family, and he was a man of influence and renown in the musical community. Much of the trumpet music of the time was written specifically for him. As sergeant trumpeter, it was up to him to license other trumpeters, giving them permission to play.
The original, and to this day main, use of the tuning fork is to provide a note for musicians to use in tuning their instruments. Its pure tone makes it easy to match. The most common pitch is A (440 Hz), which is the note orchestras tune to.
Grasp the base of the tuning fork somewhat loosely between your thumb and first finger. If you hold it too firmly, you will prevent it from vibrating. For the same reason, do not hold or touch the prongs. Tap one of the prongs on a surface. Your knee is a good choice. The harder the surface, the louder the tone. Don't tap too hard or on a very hard surface or you may damage the tuning fork. After tapping, rest the base of the tuning fork on a hard surface, like a table. This will amplify the sound.
Most tuning forks are made of steel. When tapped, the two prongs vibrate, producing sound waves. The length of the prongs determines the pitch. The pure tone of the tuning fork differs from a string, which when plucked, sounds with the tone it's tuned to, but also resonates with overtones, which are tones associated with the main tone.
Tuning forks can be used to test hearing. They can also be used to detect stress fractures in bones. A tuning fork that causes an increase in pain when held against a suspected fracture indicates that there may indeed be a fracture. Some watches contain tiny tuning forks, and tuning forks are sometimes used in alternative healing.
Tuning forks can be purchased in music stores or online for under $10. Make sure you choose the right note for your purpose.