An absolute electrometer of the attracted-disk type is described . It is suitable for the measurement of alternating voltages up to 275,000 volts effective value with an accuracy of a few hundredths of a percent. A set of equally spaced coaxial guard hoops, maintained at equally spaced potentials, serves to produce a uniform field at the disk in spite of the large separation (110 cm) required to avoid sparkover at the high operating voltage. Formulas are derived by which corrections can be applied for any deviation of the individually measured hoop potentials from the ideal equal spacing.The disk hangs from one arm of a delicate balance which serves to measure the force of attraction. Light reflected from a mirror carried by the balance beam serves to magnify its motion and to indicate to the operator at a safe distance when a condition of equilibrium is reached. It also serves to indicate the height of the disk relative to its surrounding guard ring. The scale reading corresponding to the ideal coplanar condition is obtained from auxiliary measurements made with a pair of special microscopes adapted for measuring distances in the line of sight by a calibrated focus adjustment.The change in the attractive force as the disk moves away from the coplanar position has been measured and compared with that calculated theoretically. The effects of this change on the sensitivity and on the stability of the balance are worked out in detail.Trials of this instrument under various conditions, both normal and with certain adverse influences exaggerated, over the range from 10,000 to 100,000 volts, indicate that the probable error of values obtained with it is about 0.01 percent, and that this error will not be greatly increased when the instrument is used at 275,000 volts.
Two entirely distinct methods available at the National Bureau of Standards for the measurement of the effective value of alternating voltages in the range from 10,000 to 100,000 volts are described. A comparison of these methods over a wide range of conditions involving 64 independent det erminatiolls has shown ih em to agree within 0.02 percent.
3.3 seconds. This result, with the calculated value 12 per cent higher than the measured value, is typical of the time constants that have been checked. The error has generally been of the order of 10 or 15 per cent, with the calculated result being high. It should be emphasized that the foregoing results were computed by starting with the a-c saturation curve of the core, and that no experi mental corrections were used at any stage of calculations.If desired, the path of a transient that spans a markedly nonlinear portion of the flux-versus-current graph can be computed graphically by a step-by-step method. The main error here is caused by the uncertainty introduced into the initial condition by the hysteresis effect.If a comparatively large control voltage is used to force the control current to change rapidly to a point well above the knee of the control-circuit magnetization curve, the i m R e drop can be neglected during most of the change in flux, and the time required for the flux linkages to change an amount Αψ <: is only slightly greater than A\p e /E c , where Έ ϋ is the suddenly-applied control voltage. For example, reactor 66G912 connected in the self-feedback circuit was started at ψ £ = 6 weber-turns (I ae =0.8 ampere), and an electromotive force of 47 volts was suddenly applied to the control winding. This was sufficient to produce a steadystate control current of 300 milliamperes, which is well above the knee of the curve of Figure 11. The final value of φ £ is, from Figure 11, about 21 weber-turns (7 ER =10.2 amperes). Therefore, the approximate time of rise is A^c/jE'C=(21-6)/47 = 0.32second.An oscillogram showed that the load current rose almost linearly to 10 amperes in 0.31 second.
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