The regulator (or regulator arm) is part of a wristwatch movement. It is a pointer that is attached to the balance cock. By moving the pointer towards “+” or “-” (or “A” or “R”) the balance spring is shortened or extended by a minimum amount. As a result, the rate of the watch is speeded up or slowed down.
Invented and patented in 1755 by Joseph Bosley. From his invention almost every later regulator design that followed is derived.
Reed's Whiplash Regulator
In 1867 the American George P. Reed invented and patented an improvement to the Bosley regulator, US patent No. 61,867, dated February 5, 1867.
Today this adjuster type is mostly called swan's neck regulator or swan neck device, because of the shape of the curved spring which is vaguely in the shape of a swan's neck.
Please note that the term Regulator watch is a different use of the word “regulator”.