此款極座標式求積儀是由美國Keuffel & Esser公司在德國製造，型號為4212，其特色為具備可延伸固定桿，可提升收納便利性。使用方式為，將可延伸固定桿展開後，放上加重塊確保使用過程中固定點不會位移，利用追蹤點沿著待測面積邊緣繞一圈後，讀取刻度數值並乘上儀器盒內提供的單位係數，即可獲得該區域的面積。

This polar planimeter was manufactured in Germany by the American company Keuffel & Esser, model 4212. It features an extendable base rod that enhances storage convenience. To use it, first, extend the base rod and place a weight on it to ensure that the referencing point does not shift during use. Then, trace along the edge of the area to be measured. After completing one full track of the perimeter of the area, read the scale value and multiply it by the coefficient provided in the instrument case to obtain the area of the region.