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One of the most convenient methods of measuring the atmospheric conductivity is that due to Gerdien. In this method a stream of air is drawn through the space between two concentric cylinders, the inner one of which is charged. By observing the fall of potential of the inner cylinder in a known time, it becomes possible to calculate the conductivity due to those ions which are of opposite sign to the charge on this cylinder. Since the theory of the instrument, as worked out by Gerdien, postulates certain conditions which are not always fulfilled, it becomes desirable to see how far violation of these conditions is likely to affect the results; this is especially so since the apparatus in its usual form is of a convenient general design, is easily transportable, comparatively simple in its action, and is adaptable to measurements of conductivity of both signs.

While ideal rotational surfaces of the pivot and the race of ball bearings can touch the spherical ball surfaces only at mathematical points, these surfaces actually “flatten” at the contact, due to elastic compression so that contact occurs over a finite area. It is shown in this paper that as a result of this flattening, owing to unavoidable small relative slipping over some of the contact areas, even the most precise and flawless ball bearings possess definite friction. This source of unavoidable friction is analyzed, and the frictional torque due to it is computed for an individual ball, for the whole bearing, and for both bearings holding a rotor or gimbal.

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