The absolute measurement of inductance . wherep — 2?r times the frequency of the alternating , _R \Vn -s- or the volt-drop may be kept constant and the current measured ineach case. Then R JA-A* If the current is not of sine wave form, a correction must be applied. 2. The Method of this Paper. It occurred to us that a modification of the method quoted abovefrom Gray would be as well adapted to precision measurements asany other proposed. Instead of using the electrometer to measure thedifference of potential at the terminals of the inductive coil and ofa fixed resistance J?, w
The absolute measurement of inductance . wherep — 2?r times the frequency of the alternating , _R \Vn -s- or the volt-drop may be kept constant and the current measured ineach case. Then R JA-A* If the current is not of sine wave form, a correction must be applied. 2. The Method of this Paper. It occurred to us that a modification of the method quoted abovefrom Gray would be as well adapted to precision measurements asany other proposed. Instead of using the electrometer to measure thedifference of potential at the terminals of the inductive coil and ofa fixed resistance J?, we vary the resistance R until the differenceof potential at its terminals is equal to that at the terminals of the , ABSOLUTE MEASUREMENT OF INDUCTANCE. 127 inductive coil, as shown by an electrometer. Then, since the alter-nating current lis the same in both, and a sine wave form is assumed, JB=Is/r*+p2Z2,r being the ohmic resistance of the inductive coil. Then Z=i-s/B r\ (See fig. 1.). Fig. l.—A £=noninductive resistance; B CD=impedance triangle for inductive coil. This is an extremely simple formula, in which only two quantities,the resistance and the frequency, have to be determined resistance r is usually so small that an approximate value for it issufficient. In the simplicity and directness of the method and thesmall number of quantities to be determined lie the advantages of thisover other methods. The chief objection to this method is that it is necessary to have aperfect sine current, or to know the exact form of the current wave inorder to calculate the correction due to any harmonics that may bepresent. So far as we know no accurate determination of inductanceby this method has ever been published, and probably because of thisrequirement. Most alternating current generators yield currentshaving harmonics of considerable magnitude, and the wave form ofcourse varies according to the load. It is necessary, therefore, todetermine the
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