Young Blonde Girl Wearing Colorful Hair Rollers In The Kitchen

The diameter of a roller varies from approximately 0.8 inches (20 mm) to 1.5 inches (38 mm). The hair is heated, and the rollers strain and break the hydrogen bonds of each hair's cortex, which causes the hair to curl. The hydrogen bonds reform after the hair is moistened.
A hot roller or hot curler is designed to be heated in an electric chamber before one rolls it into the hair. Alternatively, a hair dryer heats the hair after the rolls are in place. Hair spray can temporarily fix curled hair in place.
In 1966, Panasonic produced the first electrically-heated hair rollers, and released them in Japan.