Planimeters
Polar–Coradi

A planimeter with a closed curve.

Instrument maker Gottlieb Coradi established a workshop in Zurich in 1880 and soon began making polar planimeters in the Amsler style. In 1894 he adjusted the pole arm so that it was higher than the tracer arm. He also replaced the pivot connecting the arms with a ball joint. The result, which he called a "compensating" polar planimeter, prevented errors introduced by planimeters made in the Amsler style because Coradi's instrument could trace in both the clockwise and counterclockwise directions. As the relatively large number of Coradi planimeters in the collection suggests, the instruments were widely sold in the United States in the early 20th century—even though they were somewhat more expensive than Amsler planimeters. Because the firm put a label inside each case with the exact date on which each planimeter was manufactured, along with the specific calibration for each instrument, it is easy to identify Coradi planimeters and arrange them in chronological order.