此款徑向式求積儀是由美國Keuffel & Esser公司製造，型號為4215。徑向式求積儀可測圓形圖表的平均半徑，可用來計算紀錄於圓形壓力圖的流體平均流速。該求積儀的使用方式為，將金屬固定銷固定在圖形中心，求積儀的滑槽放於固定銷上，利用追蹤點沿著圖形邊緣繞一圈，讀取刻度數值並乘上儀器盒內提供的單位係數，即可獲得該區域的平均半徑（例如平均流速）。

This radial planimeter was manufactured by the American company Keuffel & Esser, model No. 4215. The radial planimeter can measure the mean radius of circular charts and is used to calculate the average fluid flow rate recorded on circular pressure charts. The method of use involves fixing a metal pin at the center of the chart, placing the planimeter’s slider onto the pin, and tracing the edge of the chart with the tracing point. After completing the trace, the scale reading is multiplied by the coefficient provided in the instrument case to obtain the mean radius (e.g., average flow rate) of the area.