An electrostatic voltmeter

An electrostatic voltmeter possessing several unique features is described. The size is a minimum for such instruments, since the potential difference is applied to two electrodes, the geometrical arrangement of which is analogous to concentric spheres. The inner electrode is supported on the end of an insulating bushing introduced through the outer electrode, which is a metal can. The inner electrode contains a suspended dumb-bell rotated by electrostatic repulsion, as in the Coulomb torsion balance. The deflection is read by means of a mirror, which, for convenience in mounting the instrument at a safe height, may be placed at the lower end of the insulating bushing. For purposes of automatic control, the dumb-bell may be connected mechanically with an apparatus at ground potential. The design may be adapted to the use of high gas pressures, desirable in the measurement of very high voltages. The sensitivity of the instrument is discussed and calibration curves given for various suspensions. So many and such varied types of electrostatic voltmeters are described in the literature that the value of publishing a new design may well be questioned. However, it is a matter of common observation that the design for a simpler, less expensive, or more easily constructed instrument is always of considerable interest to investigators ; this is probably chiefly due to the fact that the average worker spends a considerable share of his time and energy trying to improve the design of his apparatus. Accordingly, an electrostatic voltmeter is here described which seems to possess several points of unique interest. The writer has used instruments of this design for several years with entire satisfaction. Many of the details are not original, the ideas being accumulated in the X-ray laboratories at Stanford, Cornell, and Columbia. In Figure 1 is shown a vertical section through the instrument. A circular metal base B is supported on three insulators A by means of leveling screws. Resting on B and inclosing the system C is an inverted metal can H which protects C from dust and air currents and fixes the electrical characteristics of the inside of the instrument (that is, if the can were omitted, the electrical fields near O would depend to some extent on the distribution of charge over the walls of the room and the furniture, and this distribution would be expected to vary with weather conditions and arrangement of furniture). Through the center of B passes an insulating bushing / which supports the moving system. This latter is hung from the suspension S, and consists of a dumb-bell K, a stiff wire W passing down through the center of the insulating bushing, a disk V immersed in a light oil for damping, and a mirror M, by means of which the displacement of the moving system may be read optically. The moving system should be of reasonably light weight and small moment of inertia in order to reduce strains on the suspension, and to minimize the period of oscillation.