Computing Area Using Planimeter

For measuring area of plan Planimeter is a mechanical instrument used. Commonly used planimeter is known as Amsler planimeter as shown in figure.Its construction and uses are described in this article.

                                                      Planimeter

 The necessary parts of a planimeter are following:

1. Anchor: with a fine anchor pin it is a heavy block at its base. At a desired point on the plan it is used to anchor the instrument.

2. Anchor arm: It is a bar with one end connected to anchor block and the other attached to an integrating unit. Generally its arm length is fixed but some planimeters are provided with variable arms length also.

3. Tracing arm: It is a bar carrying a tracer point at one end associated to the integrating unit at the other end. By a hinge the anchor arm and tracer arms are connected. By means of fixed screw and slow motion screw the length of this arm may be varied.

4. Tracing point: This is a needle point associated to the end of tracer arm, which is to be moved over the outline of the area to be measured.

5. Integrating unit: It consists of a disc and hard steel roller. The axis of roller coincides with the axis of tracer arm therefore it only rolls at right angles to the tracer arm. Roller carries a concentric drum which has 100 divisions and it is provided having vernier to read tenth of roller division. By one division an appropriate gear system moves a pointer on disc for every one revolution of the roller. The reading on the integrating unit has 4 digits as the disc is provided with 10 such equal divisions:

A.      Unit read on the disc

B.      hundredth and Tenth of a unit read on the roller

C.      Thousandth read on the vernier.

Thus, reading on roller is 42 and vernier reads 6, if reading on disc is 2 then the whole reading

                                                                 F = 2.426