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CHAPTER II.

The Polar Planimeter.

Pole and Its Forms.

In the Ball Pole Planimeter, which is the one we are discussing, the end of the Polar Arm P carries a short rod of polished Steel permanently attached to the Arm. This short rod terminates in a ball which fits accurately into a conical receptacle in the top of the Pole Weight and without shake or play, the ball and its receptacle forming a fixed center around which the entire instrument can freely turn.

The Pole Weight carrying this conical receptacle may be either round or square in shape and is usually made of some heavy metal such as lead or brass to prevent any accidental movement of the Pole while operating the Planimeter.

In some forms of the instrument the Pole is held more securely in its receptacle by means of a small cylindrical weight which is slipped over the prolongation upwards of the short steel rod already mentioned. In the form under discussion the Polar Arm is extended beyond the Pole and carries the Weight B at the further extremity of this extension as shown on the drawing. This arrangement not only serves the desired end of holding the Pole securely in its seat but has the further advantage of acting as a balance to the weight of the entire instrument and relieving the Wheel of all unnecessary pressure on the paper.

The other end of the Polar Arm is connected with the frame or Carriage C of the Planimeter by a fine pivot-joint at F which allows of perfect freedom of movement of Tracer Arm and Carriage about F as a center.

From this description it will be seen that the Tracer T revolves about the Carriage Pivot F as a center, while the Pivot F, and in fact, the instrument itself revolves about the Pole P as a fixed center with the length FP of the Polar Arm as constant radius.

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