What phenomenon is described when heat is concentrated at the tip of an insulated needle?

The phenomenon described when heat is concentrated at the tip of an insulated needle is known as the point effect. This effect occurs because the small surface area at the tip allows for a significant concentration of heat when current passes through the needle. The lack of surrounding material results in little heat dissipation, leading to a very high temperature at the needle's tip. This is particularly relevant in procedures such as electrology, where localized heat is necessary to effectively treat hair follicles.

In other terms, the point effect maximizes the heating effect due to the geometry of the needle, allowing for efficient and precise hair removal.