. Determination of the ratio of transformation and of the phase relations in transformers . nd a flat wave will givea higher ratio. As the exciting current enters merely as a cor-rection to the ratio, and as the w^ave form only slightly altersthe exciting current, the effect of vv^ave form will necessarily beslight. Rosa. ILloyd. J Ratio and Phase of Transformers. II POTENTIAL TRANSFORMERS. The ratio of a potential transformer is determined by meansof a differential dynamometer voltmeter. In this instrument thetorque due to one set of coils is balanced against the torque due tothe other set. B
. Determination of the ratio of transformation and of the phase relations in transformers . nd a flat wave will givea higher ratio. As the exciting current enters merely as a cor-rection to the ratio, and as the w^ave form only slightly altersthe exciting current, the effect of vv^ave form will necessarily beslight. Rosa. ILloyd. J Ratio and Phase of Transformers. II POTENTIAL TRANSFORMERS. The ratio of a potential transformer is determined by meansof a differential dynamometer voltmeter. In this instrument thetorque due to one set of coils is balanced against the torque due tothe other set. Bach set of coils consists of a pair of fixed coils andone moving coil between them. The two moving coils are rigidlyconnected, one above the other, but have separate leading-inwires. In determining the ratio of the primary and secondaryvoltages of a transformer the coils of each set are connected inseries with each other and with a large noninductive pair is supplied with current from the primary terminals, theother from the secondary terminals. Then, if each pair has the. Fig. 2. same constant (that is, if the torques are equal and opposite forequal currents in the coils), a balance is obtained when the resist-ances of the two circuits are proportional to the respective electro-motive forces. To prevent interaction between the two sets ofcoils, the moving coil of one set is in the plane of the fixed coils ofthe other set. In other words, the two moving coils are mountedon the suspended system at right angles to each other. A cur-rent of about ampere in each system gives sufficient sensi-bility so that a change of one part in five thousand may bedetected. In using the instrument, the primary emf. is applied througha suitable resistance to one set of coils; to the other set first theprimary and then the secondary emf. is applied; the resistance 12 Bulletin of the Bureau of Standards. ivoi. 6, No. i. being adjusted each time for a balance (see Fig. 2). The ra
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