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Various Forms of Planimeters and their Characteristics.

Precision Rolling Planimeter.

No. 8.

That form of Planimeter which may be considered as best representing the Precision class of instrument is what is termed the “Precision Rolling Planimeter” by its maker Mr. Coradi of Zurich detail drawings of which are given on Plate XI.

It will readily be seen from the drawings and from the following description of the characteristic features of its construction that the accuracy of the instrument and its operation is entirely independent of the nature or condition of the surface upon which it works, enabling measurements to be made with it of the areas of figures drawn on poor paper, tracing cloth or other material with the same degree of accuracy as though the surface were of the most favorable character.

By referring to the drawing it is seen that the Planimeter has three points of support, the two wheels or rollers R and R' and the tracer rest Z. These wheels or rollers are of exactly the same size and are rigidly attached to an axis which carries the frame and recording apparatus of the instrument. The bearing surfaces of these wheels are covered with fine slightly projecting points or teeth which entirely prevents any slipping whatever of the wheels on the surface over which they roll. The rigid connection of the wheels with the axis which turns with them prevents any movement of the Planimeter in any direction other than backwards or forwards along a straight line at right angles to the axis— this straight line being the “base” of the instrument and differing from the base of the Polar form, which it will be remembered is a circle.

One of these large wheels is provided with a fine toothed rack F in which engages the teeth of a smaller wheel E fixed upon the steel axis B, the end of the axis carrying the spherical segment M. The spherical segment M is of very finely hardened polished steel and when in use is lowered until it comes in contact with a polished steel cylinder C, to which it imparts its motion by frictional contact. To this steel cylinder is attached the recording mechanism of the Planimeter W, V and O which by means of the parts mentioned thus records the number of revolutions and fractions of a revolution of the wheels R' due to any movement of the Planimeter in the direction of the base.

The Tracer Arm T is graduated for its entire length having the zero of its graduation at the tracing needle D. This arm slides in sleeves in the frame and can be adjusted to any given length or Setting by means of the Vernier P. When desired an extension of the Tracer Arm is furnished by means of which its length can be greatly increased, thus adding to the size of the area which can be measured with the instrument in one operation.

The range of the Tracer Arm is about 30 degrees on each side of the base or center line which with the usual length of Tracer Arm allows of the measurement of an area 50 centimeters in width and of unlimited length being performed in one operation— a capacity not possessed by any other form of Planimeter.

This instrument is made in two sizes— the smaller size having length of Tracer Arm of 20 cms. and the larger of 30 cms., which lengths can be increased by means of the extension mentioned above to 40 cms. and 50 cms. respectively.

The high degree of accuracy of this Planimeter together with its capacity and independence of the nature of the surface on which it operates gives it the highest place in the class of instruments to which it belongs.

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