This unit is designed to determine the time period of spiral spring and its frequency. Spiral spring is a type of spring which is formed by a metallic strip wound around a rod at same point.

The apparatus consists of a shaft on which the spiral spring is attached. A metallic rod, containing masses at its edges, is used to deflect the shaft. The deflection can be measured through a protector and the extent to which the spring was deflected initially can also be found.