Abstract :
IN a direct-current measuring instrument, the deflection is usually proportional to the strength of the current to be measured, and it is also proportional to the strength of a magnetic field, produced by permanent magnets. In the corresponding alternating-current instrument, the magnetic field must ordinarily be produced by the measured current itself, and the deflection will therefore be inherently quadratic with respect to the measured quantity. In a thermal instrument, the heating effect of the current is observed, and the scale is likewise quadratic. This correspondingly holds for electrostatic alternating-current instruments. With this inevitable second-power relation follows uneven scale divisions, and poor sensitivity at low values, two circumstances which have a decidedly detrimental effect upon the accuracy of alternating-current measurements. In work requiring a high degree of precision, some indirect method is therefore often employed, and the alternating-current measurement is reduced to a direct-current measurement by use of an intermediate apparatus, which may be generally called a comparator. By aid of the comparator some effect of the alternating current to be measured is either observed, through direct-current measurements, or neutralized by means of a direct current, the exact value of which may be determined with the extremely high accuracy of the direct-current potentiometer.
