Helmholtz coils

If the number of turns in the Helmholtz coil (R) is doubled, how will the radius of the electron beam (r) change ... 

Measure the diameter of one of the Helmholtz coils. Divide the diameter by 2 to get the radius. Record the radius (R), in meters, in Data Table 1. The number of turns N should be written on the coil. Record N in Data Table 1. Calculate the constant k using the equation given in the introduction, and record the value in Data Table 1. 

Assemble the Helmholtz coils apparatus around the electromagnetic tube. The entire arrangement is shown in Figure B. Ask your teacher to inspect... 

R Radius of Helmholtz coil (m) N Number of turns in coil k... 

V Accelerating voltage (V) 1 Current in Helmholtz coils (A) r Radius of electron beam path (m)... 

Measuring and Using Numbers Using the values for the current in Helmholtz coils (/) from Data Table 2, the value for k from Data Table 1, and the equation B = kl, calculate the values for magnetic field strength (B) and enter them in Data Table 3. Then calculate the square of the field strength (B2) and enter the values in Data Table 3. 

In our final realization (Fig. 18), the probes use the Helmholtz coil geometry, favoring ease of use and efficient sample temperature control over a wide range of temperature values. The tunable, broad-band probe is inserted into the magnet from below and fixed to the bottom part of the magnet assembly in a simple way reminiscent of most high-resolution NMR systems. Thanks to this design, it is possible to use standard 10 mm NMR sample tubes which are inserted comfortably from above without any need to manipulate the probe. 

Thomson et al. reached the transform limit by incorporating two improvements.21 First, Helmholtz coils were placed around the interaction region to cancel the earth s magnetic field. Second, the time interval allowed for the collisions had previously been defined by the laser pulse and the field ionization... 

In the conventional measuring mode the sample stays in the NMR tube, and thus in the radiofrequency Helmholtz coil all of the time. In the continuous-flow mode it resides within the NMR detection coil only for a distinct time of some few seconds (Figure 1.2). This residence time t is dependent upon the volume of the detection cell and the employed flow rate (Table 1.1). For example, a detection volume of 120 jjlI, together with a flow rate of 0.5ml/min, results in a residence time of 14.4 s, while with a detection volume of 8 jjlI the residence time is only 0.96 s. A shorter residence time t within the NMR measuring coil results in a reduction of the effective lifetime of... 

Figure 7.2.2 Schematic diagram of the flow probe developed by Dorn and co-workers and used for the direct coupling of SFC to NMR (a) insulated glass transfer line (b) glass insert (c) Cu/constantin thermocouple (d) stainless steel equilibrium coil (e) brass shield (f) Helmholtz coil (g) ceramic flow cell (h) brass Swagelok fitting. Reprinted with permission from Allen, L. A., Glass, T. E. and Dorn, H. C., Anal. Chem., 60, 390-394 (1988). Copyright (1988) American Chemical Society...

Fig. 5.10. Apparatus, not to scale, of Jones et al. (1993) for positron impact ionization of atomic hydrogen. Squares with crosses, Helmholtz coil shaded rectangles, stainless steel shielding black rectangles in beam line, lead shielding.

Nineteenth-century records report successful electrochemical healing of broken bones (Stevens, 1812). The beginnings of a modem phase in this work are attributed to Brighton at the University of Pennsylvania (1966). The technique has been developed so that it is an accepted method in orthopedics. The beginning of a noninvasive technique using a Helmholtz coil to induce currents is attributed to Pilla (1974). Both dc and ac currents have been used. Typically, the methods employ pulses lasting 5 x 1(T3 s with a repetition rate of 15 per second. 

The equipment to carry out this test consists of a pair of Helmholtz coils, which creates the non-uniform magnetic field, and a small, nonmagnetic sample holder, included between the coils and... 

One of the first things one notices about an EPR spectrum is that it is a first-derivative spectrum rather than the more typical absorption presentation. This is due to an instrumental artifact. To enhance the sensitivity of the EPR spectrometer, the magnetic field is modulated. To obtain field modulation, a small set of Helmholtz coils are place about the sample in line with the external field. These coils allow the amplitude of the external field, to change by a small amount ( 0.01 20G) at a frequency of 100 kHz (smaller frequencies can also be used, but are less sensitive). Because the spectrometer is tuned to only detect signals that change amplitude with field changes at... 

The other two continuous spatially resolving methods, scanning LEED and scanning photoemission spectroscopy, were developed by Ertl et al. and can be applied only under HV conditions. In a scanning LEED setup, the system is configured similar to a normal LEED experiment. Two Helmholtz coils, however, are added to allow scanning of the primary electron beam over a certain fraction of the surface (50,53). One hundred data points are recorded within 10 s over a scanning area of 4 x 7 mm. The spatial resolution obtained in such an experiment is limited by the diameter of the incident electron beam. 

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