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Pla-nim′e-ter.

An instrument for ascertaining the contents of irregular plane figures.

More than thirty years ago, Oppikoffer, of Berne, invented an instrument of this description, which seemed to fulfill all the requirements of the case; but its cost and the practical difficulties attending its use prevented its general adoption.

More recently, Amsler-Laffon of Schoffhausen devised a simpler and much less costly instrument of the kind, which was exhibited at the Paris Exposition of 1867. The principle of each is nearly the same, the area being measured by a roller of given surface, the number of revolutions of which are indicated by a disk.

Amsler's polar planimeter has a sliding arm a and a pivoted arm b, the former having a pin for penetrating the paper, and the latter a tracing-point. The arm a is graduated to indicate acres, square inches, and square feet. In use, the needle-point is pressed into the paper into a point either within or without the figure; the arm a is adjusted by sliding in or out until the mark for acres, inches, or feet, as may be required, corresponds with a mark on the box c; the tracing-point is moved along the outlines of the figure until the point is reached at which it was started. If the stationary needle-point were placed without the figure, the readings of the counting-wheel d and index-roller e, subtracted from their readings at the commencement of the operation and multiplied by the indication to which the slide was set, give the area; if the stationary point be without the figure, a number opposite the indication on the slide is added to the final readings on the wheels d e, and the former reading subtracted therefrom; the difference is then multiplied, as before.

Other planimeters have been constructed by General Morin and by Mr. Sang. Sometimes called a platometer.

1877.

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Sang (1)
M. Morin (1)
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