Some recent advances in alternating-current measuring instruments

The writing of this paper has been prompted by the large amount of valuable material which has been produced during the last few years, but little of which has been brought into such a form as to make it available to the designer of electrical measuring instruments. By a process of elimination, the operating principles employed are gradually becoming less in number. For example, the use of the hot-wire principle is now almost confined to radio-frequency measurements, and the prophecy is hazarded that for the measurement of current and voltage at ordinary frequencies the electro-dynamic and induction principles will, before long, give place to the moving-iron principle. A useful feature possessed by the moving-iron type of meter is the wide latitude which is possible as regards the opening out or closing in of the scale divisions to suit the purpose for which it is required. The moving-iron instrument is investigated, from what is thought to be a new standpoint, and a form of ? precision? moving-iron ammeter and voltmeter, with an electrical accuracy of about 0.1 per cent and suitable for use indiscriminately with alternating current and direct current, is described. The use of series condensers for the extension of the range of electrostatic voltmeters for extra-high-voltage measurements is dealt with, the conditions necessary for the attainment of reasonable accuracy being considered. The use and limitations of thermo-expansion (hot-wire) ammeters for radio-frequency current measurements are considered, and a current transformer suitable for such measurements up to 1 000 amperes is described. It is shown that special considerations are involved in the measurement of rectified current. Some recent developments in the field of graphic instruments are dealt with, particularly as regards the ?relay? pattern which has proved to be valuable for traction and many other special purposes. A method of ensuring constant frequency on a supply system is considered, together with the advantages which accrue, particularly as regards ease of interlinking, and the possibility of driving recorders, time switches, etc., bymeans of a simple and compact self-starting synchronous motor. The design of current transformers is dealt with in detail, including the use of some new materials for the core, permeability and core-loss curves being given. The design of a bar (bushing) current transformer for lowcurrents as well as of a precision type of current transformer is described. The latter is suitable for use with sub-standard measuring instruments, and has a ratio error of less than 0.1 per cent and a phase displacement of less than 5 minutes. The desirability of connecting a protective resistance in series with voltage-transformer fuses is emphasized, and it is shown that such a resistance, if properly designed, need not adversely affect the characteristics of the transformer. The capacity of meters, shunts, current transformers, etc., to withstand heavy short-circuit currents is considered, and safe limiting values are deduced.