CHAPTER II.
The Polar Planimeter.
Polar and Tracer Arms.
Both Polar and Tracer Arms are usually square in section and either made
of rectangular tubing or have the under part of the Arm cut away in order
to make them as light as possible consistent with the requisite strength
and rigidity. One end of the Polar Arm P terminates in what is called
the Pole of the instrument, and Polar Planimeters are usually
divided into two classes, termed respectively Needle Pole Planimeters
and Ball Pole Planimeters according to the form and arrangement
of this end of the Polar Arm. Of the two forms of Pole the Ball Pole is
the one adopted for the finer grades of Planimeters; the Needle Pole being
used generally in those instruments which do not have the graduated Tracer
Arm and on which only simple ratios are indicated as will be explained
later.