from which
K1 = SKR ÷ S1R
That is to say, if the respective resistances corresponding to the two scales be known, and the constant of the high scale be determined by comparison with a standard, then by aid of these known values and the maximum readings on the two scales, the constant of the low scale may be calculated. It is also possible to calculate the constant of the high scale if the constant of the low scale be known, together with the values of the resistances corresponding to the two scales; for from the equation previously given.
K = RS1K1 ÷ R1S
Ques. What is a frequent cause of error in an alternating current meter, and why?
Ans. The deterioration of its insulation, which permits the working parts of the instrument coming in contact with the surrounding metal case.
A convenient method of testing for deterioration of insulation is shown in [fig. 2,905].
Fig. 2,907.—Diagrams showing various synchronous converter transformer connections. The diametrical connection is used most frequently as it requires only one secondary coil on each transformer, this being connected to diametrically opposite points on the armature winding. The middle points can be connected together and a neutral obtained the unbalanced three wire direct current having no distorting effect. With diametrical secondaries, the primaries should preferably be connected delta, except with regulating pole converters where they must be connected Y. Diametrical secondaries with delta primaries should not be used with regulating pole converters. Double star connection of secondaries may, however, be used with delta primaries, and is free from the trouble of the triple harmonic of the transformer appearing in the primary. In this case, however, the two secondary neutrals must not be connected with each other.