CHAPTER II.
The Polar Planimeter.
Essential Parts of the Instrument.
The Polar Planimeter, Fig. 1, Plate I,
consists essentially of the arm
FP, known as the Polar arm,
the Wheel W termed the
Integrating Wheel, and the graduated
arm RS known as the
Tracer Arm, all of polished german silver.
The lengths of these two arms as well as the diameter of the wheel are
entirely arbitrary and differ slightly in Instruments of different makes.
Nor is it essential that the dimensions of these parts should bear any
definite relation to each other than that which experience has proved to
be the most suitable for any given form of instrument. The usual length
of both Polar and Tracer Arms is from six to eight inches with a diameter
of Integrating Wheel of perhaps three-quarters of an inch, these dimensions
having been found to be the most convenient.