Determination of the constants of instrument transformers . siderable caremust be taken to keep the leads twisted together to avoid induc-tion from stray fields. With the dynamometers used, a maximum sensibility, in thecase of good transformers at full-load current, of about one partin 20,000 in ratio and in phase angle was obtained by usingfrom to volt drop in the shimts, but of course the finalaccuracy is less. This requires but a few hundredths of an ohmto be placed in the 5-ampere secondary of the transformer, whichis comparable to the resistance of connecting leads as ordinari
Determination of the constants of instrument transformers . siderable caremust be taken to keep the leads twisted together to avoid induc-tion from stray fields. With the dynamometers used, a maximum sensibility, in thecase of good transformers at full-load current, of about one partin 20,000 in ratio and in phase angle was obtained by usingfrom to volt drop in the shimts, but of course the finalaccuracy is less. This requires but a few hundredths of an ohmto be placed in the 5-ampere secondary of the transformer, whichis comparable to the resistance of connecting leads as ordinarilyused. The energy necessary to operate the instruments is takenchiefly from the primary instead of from the secondary. As the sensibility in making the ratio settings varies inverselyas the square of the current, dynamometers having two currentranges were ordinarily used in order to keep up the sensibility atlow loads. The sensibility of the measurement of phase anglevaries inversely with the load. 296 Bulletin of the Bureau of Standards. {Vol. 6. No. 2 3 4 SECONDARY AMPERES Fig. 26.—Showing effect of magnetic history on the ratio of current transformer A. 340 20 ^^ <:v. ^ *=*«.^ ^5j^ ^ ^ ^^ ^ ^ ^ === Fig. SECONDARY AMPERES 27.—Showing effect of magnetic history on phase angle, current transformer. J Instrument Transformers. 297 The multiplier of the quarter phase instrument was usuallyadjusted to give a deflection of i centimeter per millivolt, and thecalibration was made by introducing the emf. at the terminals ofa resistance of ohm placed in series with the field coil, asexplained above. In Figs. 12 to 25 are shown ratio and phase angle curves fordifferent types of secondary loading. For convenience of refer-ence both the actual ratio and the ratio expressed in percentageof nominal ratio are given on the vertical scale. TABLE for Current Transformers. Trans-former Primarycurrent No
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