Planimeter
Fig. 52. General Construction of Planimeter
The planimeter is an instrument for measuring areas in general, and especially for measuring the areas of indicator cards. Some forms give the mean effective pressure directly, without computations, by changing the scale to correspond with the spring used in the indicator. A planimeter of this type is shown in [Fig. 52]. The method of manipulating this instrument is as follows. Set the arm BD equal to the length of the card EF, by means of the thumb screw S, and set the wheel at zero on the scale, which must correspond to the spring used in the indicator. Next, place the point D at about the middle of the area to be measured, and set point C so that the arm CB shall be approximately at right angles with BD. Then move D to the upper left-hand corner of the diagram, and with the left hand move C either to the right or left until the wheel comes back exactly to the zero point on the scale; then press the point firmly into the paper. Now, go around the outline of the diagram with point D from left to right, finishing exactly at the starting point. The mean effective pressure may now be read from the scale opposite the edge of the wheel.
When very accurate results are required, the tracer point D may be passed over the diagram several times, and the reading divided by the number of times it is thus passed around. With short cards, 3 inches and under in length, it is best to make the arm BD twice the length of the card, and go around the diagram twice, taking the reading directly from the scale as in the first case.