Final answer:
The thermal perm, also known as a hot perm, utilizes preheated clamps to set hair into curls, and was a predecessor to modern chemical perms.
Step-by-step explanation:
The name of the perm that used clamps, which were preheated on a separate electric unit and then placed over the hair, is called a thermal perm or hot perm. This method involves using heat to create and set the curls. Hair is wrapped around rods or rollers, and then heated clamps from an electric heater are applied. The heat helps to create a lasting curl. Thermal perms were more common before the arrival of modern chemical perms which use chemicals to break and reform the bonds of the hair.