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CHAPTER II.

The Polar Planimeter.

Influence of Surface on Wheel Movement.

As the revolution of the Integrating Wheel and hence the entire working of the Polar Planimeter is due to the frictional contact between the rim of the Wheel and the surface of the paper or other material upon which the Wheel moves, it is evident that the nature of this surface must be a matter of the greatest importance in every case where a high degree of accuracy is desired. One great advantage possessed by the Rolling Spherical Planimeter and the Precision Disk Planimeter and other of the higher form lies in their complete independence of the nature of the surface upon which they are operated. In the case of the Rolling Spherical Planimeter the movement is produced by very heavy toothed wheels which roll over the surface and on whose motion inequalities and irregularities which would be fatal to the accuracy of the Polar Planimeter have little or no appreciable effect, while in the Precision Disk form of the Integrating Wheel rolls over and receives its motion from a polished metal disk of uniform surface and free from any injurious influence.

In the case of the Polar Planimeter uniformity of surface is a very strict essential to accuracy since the movement of the wheel which is partly slipping, partly rolling over the surface should have identically the same conditions of surface at every point in its path in order that the influence due to the surface may be of the same kind and extend throughout its entire operation. In fact uniformity of surface is of more importance than the nature of the surface itself, since very accurate results are obtainable with any of the papers or materials ordinarily used for drawing and map making purposes. Those drawing papers having a medium smooth surface finish give perhaps the best results and the paper need not be glued or stretched on the board provided it lies smoothly and without wrinkles.

Care should be taken that the drawing board or table on which the paper lies should be perfectly level, true and free from any irregularities.

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