The Precision Measurement of Transformer Ratios

Techniques for comparing four-terminal-pair admittance standards

“…The calibration of th e brid ge was in some ways eve n more formidable th a n its co nstruction_ Th e first step wa s to c hec k the lin e arity of the bridge di als a t sever al fre que ncies by an external calibrati on techniqu e in which a fix ed admitta nce was re peat edl y added in paralJ el with one sid e of th e bridge for variou s se ttings of a n a uxiliary admittance in parallel with th e other sid e_ Both magnitude a nd quadrature balance linearities of all di als were c hec ked in this way_ A resolution of 1 part in 10 10 was obtain ed at 1592 Hz, and of 1 part in 10f) a t all oth e r frequencies_ The seco nd s tep was to de te rmin e th e actual brid ge se ttings whi c h would produ ce an exac t 10:1 ratio_ A modifi cation of the permutation me th od desc ribed in an earlier paper [12] was used for thi s purpose _ Th e last step in th e calibration was to de termin e the actual magnitudes and phase a ngles correspo ndin g to c ha n ges of th e real and quadrature dials of th e brid ge over their e ntire ran ges_ A quadrature bridge and a syste m for comparing admittances in a 1000:1 ratio are need ed for thi s step. Three s te p-up s usin g the 10:1 ratio of the bridge unde r tes t provided th e 1000: 1 ratio with ample accuracy_ Th e bridge accuracy is optimum at 1592 Hz, at which frequency the ratio is adjustable over a range of ± 5 parts in 10 4 in ste ps of 1 part in 10 9 for both real and quadrature co mponents.…”

“…Fortunately the current transformer part of the bridge need not be provided with an adjustable ratio, and is therefore much easier to build. A quite different approach to an equal-power, four-terminal (but not four-pair) impedance bridge has been developed by Henry Hall of the General Radio Company.2 Both two-pair and four-pair equal-power bridges have been described in the literature [12], but the operation of the four-pair version is not clear unless two-stage transformers are employed.…”

Voltage dependence of precision air capacitors

“…The values in table 4 were obtained either immediately after assembly of the capacitors, one month after assembly, or eight months after assembly. The values in table 2 were obtained eight months after assembly.…”

“…One method for measuring the voltage ratio of a transformer is to connect admittance standards to the transformer so as to form a bridge as shown in figure 1 [4]. In general, if the transformer has a nominally m: n ratio, then m + n nominally equal admittances are used , and m + n detector balances are required corresponding to a cyclic permutation of these admittances_ In this sequence of balances each admittance appears with voltage El m times, and with voltage E2 n times.…”

Some techniques for measuring small mutual inductances

“…1 Ground capacitance current betwee n the primary windings is negligible ; it is discussed in ~el' l ilJn 4 of Ihi~ p ape r . method described by Cutkos ky and Shields [2]. The bridge balance is accomplished by th e adj ustment of e3, which is an adjustable voltage source, small relative to el and e2.…”