The limitation in the null instrument sensitivity of bridges leads to errors which are due to the uncertainty in the null adjustment. This paper deals with this sensitivity error in Wien frequency bridges. Formulas for the relative error in frequency measurement and for optimum bridge parameters are derived. The influence of deviations from the optimum parameters is shown and discussed. It is shown that, for bridges covering a wide frequency range, certain free parameters should be made adjustable to maintain good sensitivity.
A simple but exact derivation of the conditions for ground potential on the null instrument input in nongrounded ac bridges is given. It is shown that not only the ``Wagner-earth,'' but also other networks and circuits between the power source and the ac bridge can easily fulfill these conditions. Advantages of such other networks and circuits are discussed. General considerations for optimum bridge details for measurements of highest obtainable accuracy are given. Practical supply circuits for ac bridges are described. Furthermore, it is shown how the adjustment of the earth capacity neutralizing system can be made in a minimum amount of time.
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