Alternating-current transformers are so useful in the measurement of current and potential, by reducing the current or potential that must be applied directly to the instruments, that they have been extensively used in engineering work for the measurement of current, voltage, and power, not only for the heaviest currents and highest potentials, but also for currents and potentials of moderate values. Such transformers, when properly constructed, can safely be em.ployed in connection with

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... on voltmicters, ammeters, and wattmeters for measurements of considerable accuracy. Indeed, if the constants of the transformers have been accurately measured, the precision of the results will depend chiefly on the indicating instruments, for the transformers themselves are more permanent and less liable to injury than the more delicate instruments used with them.