Absolute measurement of capacitance by Maxwell's method
Harvey L. Curtis, C. Moon · Scientific Papers of the Bureau of Standards · 1927
The absolute measurement of capacitance by Maxwell's method is based on the assumption that certain conditions are fulfilled by the experimental apparatus.While there are at least seven of these conditions, the most important is that the galvanometer correctly integrates the current.When large capacitances are measured, a galvanometer is required in which the current through the coil does not affect the magnetism of the permanent magnet.By placing the coil in a symmetrical position with respect to the magnet this condition can be fulfilled in most galvanometers.However, this adjustment is greatly simplified by a proper design of the galvanometer.The method requires that a condenser shall be charged and discharged at a known rate.To accomplish this, contacts of platinum dipping in mercury cups have been placed on the prongs of a tuning fork driven by an electron tube.This permits the Maxwell bridge to be balanced with the same ease and accuracy as a Wheatstone bridge.However, it is necessary to determine the frequency of the fork at the time of measurement.For this purpose the fork is compared with a freely vibrating pendulum by a method which gives an accuracy of a part in a million with observations which extend over a few minutes' time.The possibility of introducing systematic errors has been guarded against by employing two independent bridges in which the factors have been varied as much as possible.Results are given showing the accuracy that can be attained.CONTENTS Page 7. Time integral of the galvanometer current is zero.503 V. Theory of the galvanometer 507 1.Effect of magnetic material in the galvanometer coil 508 2. Effect of the 'galvanometer current on the magnetic field 508 VI.Theory of Maxwell bridge when the galvanometer does not inte- grate correctly .510 VII.Calculation of the capacitance 511 487 488 Scientific Papers oj the Bureau oj Standards ivoi.wPage VIII.Apparatus for charging and discharging the condenser 513 1. Rotating commutators