Pole faces, magnetic

Following the tolerance stack through the end assembly, the bobbin dimension of 22 mm from the outside face to the back face of the magnetic pole is analysed next. This characteristic dimension does not include the tolerance on the impact extruded pole. The pole is to be moulded into the bobbin and the pole face is considered to be part of a mould related dimension. The bobbin is injection moulded using 30% filled polybutylene terephthalate (PBT). The tolerance assigned to the bobbin dimension is 0.035 mm. 

Most spectrometers measure the magnetic field by a Hall effect probe consisting of a sensor mounted on one of the pole faces of the magnet. However, such estimates of the value of B inside the cavity are not sufficiently accurate to be used for g-factor determinations. There are two ways around this problem (i) measure the spectrum of a solid free radical such as dip-henylpicrylhydrazyl (DPPH), which has a well-known g-value (2.0028), at least once during acquisition of the desired spectrum or (ii) use of an NMR gaussmeter probe inserted in or near the cavity several times during the collection of the spectrum. 

The ability to separate ions spatially is called the dispersion of a mass spectrometer. Dispersion is simply the distance between the centers of two ion beams that differ in mass by Am at the collection plate. A simple sector instrument, where the ion beam enters and exist the magnetic field normal to the pole faces and the object and image distances are the same, is known as a symmetrical geometry analyser. Examples are shown in Fig. 8. In this case, the dispersion, D is given by ... 

The sample is taken in a glass tube which is placed between pole faces of a magnet. A radio frequency source (v = 60 mega cycles sec ) is made to fall on the sample. It is done by feeding... 

Figure 2.38 Diagram of an NMR probe. The solenoid coils on the pole faces of the magnet produce the variable magnetic field. The direction of the magnetic field through the sample is indicated by M.

Fig. 4.20. Flight tube passing through the gap of the magnetic sector of a JEOL JMS-700 instrument seen from the ESA side. The shapes of the pole pieces of the yoke and the additional blocks around the tube are designed to minimize fringing fields. In addition the pole faces are rotated to increase the mass range.

Fig. 4.24. Schematic of the Finnigan MAT 90 double-focusing mass spectrometer with rotated pole faces of the magnetic sector. Reproduced from Ref. [2] with permission. Elsevier Science, 1987.

Such a relationship (and corresponding response surface) might represent magnetic field strength as a function of position in a plane parallel to the pole face of a magnet, or reaction yield as a function of reactor pressure and temperature for a chemical process. 

The magnetic field was obtained by the use of Helmholz coils or an iron core magnetic. The former had mean coil diameters of 38 cm spaced 19 cm apart to maximize the uniformity of the field near the gage base. This gave a usable working gap of about 4 cm. The latter had square pole faces of 10.2 cm on a side spaced... 

If we examine the magnetic held from such a quadrupolar device, we will hnd that the held along the central axis is exactly zero and increases linearly to a maximum value at each pole face. (The pole tips should be hyperbolic surfaces to conform to the shape of the magnetic held, although cylindrical pole tips are often used for ease of manufacture.) Quadrupole magnets are thus characterized by the gradient of the magnetic held, dB/dr, where r is a radial coordinate and... 

H. Freeman. The main difference between it and the Scandinavian machine lies in the extraction geometry. The ion source has an exit slit instead of an exit hole. This allows much higher ion currents. Unfortunately, focusing with lenses is difficult in this case because of space charge limitations. Therefore, the shaping of the beam of the Harwell separator is done with the help of the fringing field of the magnet and ajustable pole faces at the entrance and the exit. 

Position the magnetic filter device(s) in the 3 mm-gap between the two pole faces of the electromagnet. 

Variable temperature accessories are now available with commercial spectrometers. The nature of restricted space in a magnet gap and the necessity of constant temperature at the pole faces renders temperature measurement and control a much more difficult problem than in most other kinetic and equilibrium measurements. The space requirement necessitates a flow system for heat transfer with good Dewar vessels for leads and N.M.R. probe attachments. The heat-transfer medium has... 

Typical results from an analytically based design procedure published by Howe et al. (1987) are shown in fig. 55. They relate to a motor equipped with pole-face mounted magnets, and have a specified short-time overload capacity. Howe et al. discussed the effect on magnet volume of a variable electric loading Q and variable rotor diameter D, the magnet being dimensional to withstand the overload. They show that increases in either D or Q result in a reduction in magnet volume. 

---