Excerpt from A New Method for the Absolute Measurement of Electric Quantity

In the absolute measurement of current two classes of methods are available which are capable of considerable accuracy, viz, electrodynamometer methods, in which the torque exerted between a fixed and a movable coil is measured and the current in the coils calculated from this measured torque and the known dimensions of the coil; and current balances, in which the force exerted between two coils carrying the current to be measured is balanced against a known weight and the current calculated from the weight and previously determined constants of the coils. By the latter form of apparatus measurements of current have been made in which a higher degree of accuracy has been attained than with the first-named type. There are, however, numerous possible sources of error in the use of the current balance, and, although most of these can be made small individually, they may on account of their number, introduce uncertainty in the result.

The ordinary electrodynamometer methods as well as the current balance require in effect two separate experiments, the first for the determination of the value of the current in terms of a torque or force, and the second for the recording of this current in terms of the electro-chemical equivalent of silver or of a resistance and standard cell. This second experiment may introduce additional possibilities of error, owing to variations in the strength of the current or the voltage of the standard cell, errors in the resistances of the potentiometer circuits and thermo-electromotive forces in the galvanometer and potentiometer circuits, etc., and while these errors can be made small, still their possible presence increases the difficulties of the measurements.

Although the current balance is capable of yielding a high degree of accuracy, it is not desirable to depend on the value given by a single type of apparatus, even though the experiments may be repeated with good agreement by different experimenters and with different instruments. It is highly desirable that if possible the values be checked by some other method, using a totally different type of apparatus.

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