Kent Compensating Polar Planimeter with zero set device and magnifying tracer. Metal box.
The instrument is used as follows. The tracer point is moved to follow the boundary of a region to be measured. As the tracer point moves the wheel partially rolls and partially slides on the plan, and the integrating wheel records the distance the wheel travels perpendicular to the tracer arm. Once the tracer point completes its circumnavigation of the boundary, the number of turns of the wheel is proportional to the area of the region. When the instrument is calibrated appropriately, the area of the region can be read from the scale.
With the zero set device the number of turns can be read directly. Without the zero set device the user records the original setting and subtracts that from the final setting.