Pole measure, principles

The direction of rotation depends on the direction of the current in the coil, and thus the instrument is only suitable for D.C. It is, however, possible to incorporate a full-wave rectifier arranged as shown in Figure 17.11 in order to allow the instrument to measure A.C. quantities. The quantity measured is the RMS value only if the waveform of the current is truly sinusoidal. In other cases, a considerable error may result. In principle, the scale is linear but, if required, it can be made non-linear by suitably shaping the poles of the permanent magnet. The instrument reading is affected by the performance of the rectifier, which is a non-linear device, and this results in the scale also being non-linear. The error when measuring D.C. quantities can be as low as 0.1 per cent of full-scale deflection and instruments are available for currents between microamperes and up to 600 A. 

Fig. 13.17 Working principle of the MEMS-based FDNMR spectrometer. The sample (S) sits within an RE excitation coU (C), which modulates the NMR signal at the mechanical resonance frequency of a resonator made up of the detector magnet (D) and sihcon beam (O). The detector magnet sits within an annular magnet (R) and the entire assembly is within the pole piece of an external field magnet (M). Detector magnet motion is measured using a fiber optic interferometer (F)...

Before we go on to discuss these in greater detail and how these parameters affect the data, it is important to remind ourselves how a scanning device like a quadru-pole mass analyzer works. Although we will focus on quadrupole technology, the fundamental principles of measurement protocol will be vay similar for aU types of mass spectrometers that use a sequential approach for mulliel ent peak quantitation. 

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