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In virtue of the more complete symmetry realizable when the plates to be compared are not merely equal in area but also similar in shape, this method would be specially advantageous for the investigation of the possible influence of thickness and of the smoothness of the surfaces.
When the areas to be compared are unequal, so that their centres need to be at different distances from the axis, the resistance balance of the auxiliary parts demands special attention. I have experimented upon circular disks whose areas are as 2:1. When there was but one smaller disk (6 cm. in diameter) the arms of the lever had to be also as 2:1 (fig. 1). In another
Fig. l.
experiment two small disks (each 4 cm. in diameter) were balanced against a larger one of equal total area (fig. 2). Probably this arrangement is the better. In neither case was any difference of mean pressures detected.
In the figures A A represents the needle, B and C the large and small disks respectively, D the extra attachments needed for the resistance balance of the auxiliary parts.ise superfluous additions. It would seem that a practical solution may be attained, though it must remain deficient in mathematical exactness. The junctions of the various pieces can be effected quite satisfactorily with sealing-wax used sparingly. The brass U itself is mounted at the end of a rod held horizontally in front of the observer and parallel to the direction of motion. I found it best to work indoors in a long room or gallery.