9 - Methods of measurement

Abstract

This chapter discusses few basic instruments and the more general measurements to which they can be applied. When instruments are connected into a circuit, the circuit conditions are changed. Hence, the readings given by the instruments, irrespective of the accuracy of the instruments themselves, are not identical with the actual currents and voltages, which were acting in the circuit before the instruments were inserted. Alternating currents and voltages can be measured directly by a moving iron meter, or a moving-coil meter may be used in conjunction with a suitable rectifier. The use of a single rectifier is rarely entertained at low frequencies (power and audio ranges) because the circuit to which the instrument is connected will be asymmetrically loaded. At low input voltages, the nonlinear relationship between voltage and current in the rectifier leads to a much distorted scale calibration. This method of ac measurement is satisfactory up to some 100 kHz in frequency. As the scale is calibrated in sinusoidal root mean square, however, the reading will be incorrect when nonsinusoidal quantities are measured.