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CHAPTER II.

The Polar Planimeter.

Adjustment and Care of the Instrument.

The adjustments of the Polar Planimeter are few and simple but the accuracy of both instrument and its operation depends upon the perfection of those adjustments and upon the delicacy and care with which they are effected.

That condition on which more than any other the accuracy of the Polar Planimeter depends is that the axis of the Integrating Wheel W shall be exactly parallel to the Tracer Arm RS. The importance of this parallelism is readily shown not only by actual trial but also by reference to the formulae deduced in the mathematical demonstration of the theory of the Planimeter given in a succeeding chapter and from which the effect in kind and extent of any deviation from parallelism is at once determined.

Co-important with this condition is the further one that the axis shall turn in its bearings with the most perfect freedom and delicacy possible and with a minimum amount of fricture but without play or shake of any kind.

The adjustments of the axis to cause it to conform to these two given conditions are easily effected by means of correction and binding screws in the Carriage C at both ends of the axis which give movement in any desired direction to the hardened bolts which form the bearings of the conical ends of the axis.

An attempt to minimize the errors in the operation of the Polar Planimeter due to the two causes just mentioned has resulted in the form of instrument called the Compensating Planimeter by its inventor, Mr. Coradi, by which the errors while present in the result of a single operation are practically eliminated or compensated by taking the mean of the results of two operations in which the effects due to nonparallelism are in one operation taken positively and in the second operation negatively. This instrument being the highest form of the class of Planimeter under discussion will on account of its importance in this connection be taken up and described in detail later.

After a long period of use or as the result of accident the axis of the wheel may appear sluggish and to have lost the delicate sensitiveness and freedom of movement so essential to the highest efficiency of the Planimeter. Should this be due to a dulling or injury to the conical ends of the axis the only remedy is to place the instrument in the hands of a competent instrument maker who can easily restore it to its original condition. In most cases, however, this dullness or sluggishness is due to the oil which has been applied to the bearings of the axis becoming congealed or hardened, in which case the defect is easily remedied by loosening the set screws which control the bearing bolts and introducing with a fine aluminum point a drop of benzine into each bearing and then revolving the axis quickly in both directions. This will dissolve or soften the congealed oil which can then be removed by a very soft cloth. After thoroughly cleaning the bearings a drop of the finest watch oil should be introduced to each bearing by the metal point and the axis then readjusted.

In using the Planimeter great care should be taken to avoid any injury to or bending of either Tracer Arm or Needle, as the effect of such bending would be to seriously impair the accuracy of the instrument and its results. The rim of the Integrating Wheel should be carefully guarded against injury of any kind, as the slightest dirt or even speck of rust on the rim would effectually interfere with the accuracy of its operation. For this reason the Wheel should never be turned or even touched by the finger, and after use the instrument should be carefully wiped with a very soft chamois skin and the rim of the Wheel allowed to run several times over the chamois laid flat on the table or drawing board.

While every device to increase the sensitiveness, and hence the efficiency of the Planimeter, has been utilized, at the same time due care has been taken that the strength and rigidity of the instrument has not been impaired, and that with proper care it shall be capable of long continued and effective service and with small liability to injury if used with the precautions mentioned.

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