Cathode rays, deflection

Why is a cathode ray deflected by a nearby electric charge or magnet ... 

A) Crooke s cathode-ray tube with a Maltese cross target to cast a shadow demonstrating the particle behavior of electrons B) Thomson s cathode-ray deflection experiment for measuring m/e of the electron C) Millikan s oil-drop experiment that determined the charge (e) and mass (m) of the electron... 

Schematic drawing of a cathode-ray tube. A beam of electrons is deflected by a pair of charged plates bent line), but magnetic force can be adjusted to exactly counterbalance the effect of the electrical force straight line).

Another experiment showed that cathode rays were deflected by a magnetic field. This meant that they were electrically charged particles. 

Deflection of cathode rays by an applied electric field. 

Thomsons picture of the atom emerged from his work with cathode ray tubes. It was a milestone on the road to understanding atomic structure. But it was not the only major advance to come out of cathode ray experiments. Almost every television set in existence today is a cathode ray tube. The electrons stream from the cathode and are deflected by electromagnetic coils guided by signals from the television station. When an electron hits the televisions screen, which is coated with a phosphorescent material, it produces a dot of color. The dots form the picture you see on the screen. 

Figure 10.1 J. J. Thomson s method for measuring e/m, the mass-to-charge ratio of an electron (Thomson 1897). Electrons (cathode rays) leave the cathode (C) and are accelerated towards the anode (A), passing through a small hole and a further hole in plate (D) to provide a collimated beam. It then passes through crossed electric (E) and magnetic fields (B) which deflect the beam in the same plane but in opposite directions, and strikes the fluorescent screen at the end. In (a) the strengths of E and B are adjusted to bring the spot back to zero deflection (S). In (b), with electric field only, the deflection from S (y ) is measured, and e/m calculated as in the text.

In practice, neither the absolute energies nor their ratio is determined directly. Instead, the radiation of a particular wavelength falls on a multiplier-type phototube, and the voltage developed across its load resistor is measured with.a cathode-ray oscillograph. The deflection to the oscillograph is then proportional to E ... 

Walter Kanfmarm also reported the determination of the charge-to-mass ratio of cathode rays (abont 10 emu g- ) in a paper he submitted in April 1897 (7). Kanfmarm also based his result on magnetic deflection measurements however, he concluded that the hypothesis of cathode rays as emitted particles could not explain his data. (One of the outstanding questions in the study of cathode rays in the late 1890s was whether they were particles or electromagnetic waves. Thomson and Kanfmarm were typical of their coimtrymen most British researchers leaned toward the particulate hypothesis and most Germans toward waves.) Today Kanfmarm is better known for his careful measurements of the velocity-dependent mass of the electron published over several years beginning in 1901 these results were later explained by special relativity. 

In contrast to the TEM, the scanning electron microscope (SEM) works somewhat like a TV cathode-ray tube. An electron gun is used to scan a surface with the help of deflection coils and focusing lenses in a raster pattern, and the reflected electron signal is used synchronously to generate an image of very high magnification on a screen. The SEM has been in... 

In the nanosecond (ns) time-scale the use of kinetic detection (one absorption or emission wavelength at all times) is much more convenient than spectrographic detection, but the opposite is true for ps flash photolysis because of the response time of electronic detectors. Luminescence kinetics can however be measured by means of a special device known as the streak camera (Figure 8.2). This is somewhat similar to the cathode ray tube of an oscilloscope, but the electron gun is replaced by a transparent photocathode. The electron beam emitted by this photocathode depends on the incident light intensity I(hv). It is accelerated and deflected by the plates d which provide the time-base. The electron beam falls on the phosphor screen where the trace appears like an oscillogram in one dimension, since there is no jy deflection. The thickness of the trace is the measurement of light intensity. 

Cathode rays have many applications, such as in the cathode-ray tube and the cathode-ray oscillograph. The former is a vacuum tube in which the deflection of an electron beam indicates on a fluorescent screen instantaneous values of the actuating voltages or currents. The oscillograph incorporates a cathode ray tube and is used extensively in ballistics, such as for photographying rapid events(See under Cameras, High-Speed, Photographic)... 

FIGURE B.5 A close-up of the glowing path formed by a stream of electrons near a cathode in a simple cathode-ray tube, an apparatus like that used by Thomson. Note the cathode ray and the deflection of the ray by a magnetic field. 

A television picture tube is a form of cathode ray tube in which the beam of j electrons is directed toward colored phosphors on the screen. A very low pressure is required to minimize the collisions between the electrons in the = beam and the gas molecules. Collisions and the resulting deflections of the j electrons would give a blurred, dim picture. 

Figure 4.10 A cathode-ray tube consists of an electron gun and a phosphor covered screen. Deflection coils are used to sweep the electron beam over the entire screen (a). The correspondence between the power applied to the electron gun and the light given off by the phosphor is nonlinear (b).

In the scanning mode the electron beam focused on the sample is scanned by a set of deflection coils. Backscat-tered electrons or secondary electrons emitted from the sample are detected. As the electron beam passes over the surface of the sample, variations in composition and topology produce variations in the intensity of the secondary electrons. The raster of the electron beam is synchronized with that of a cathode ray tube, and the detected signal then produces an image on the tube. 

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