The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 The Scanning Microscope 97 tion. On the way to the specimen the oscillating electron pencil passes an apertured fluorescent screen, and finally a fourth lens, which is placed immediately in front of the specimen. This lens has little influence on the scanning beam, but spreads out the secondary electrons released in the specimen over a wide area of the fluorescent screen which has a suitable positive potential against the specimen. The beam current and the secondary electr
The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 The Scanning Microscope 97 tion. On the way to the specimen the oscillating electron pencil passes an apertured fluorescent screen, and finally a fourth lens, which is placed immediately in front of the specimen. This lens has little influence on the scanning beam, but spreads out the secondary electrons released in the specimen over a wide area of the fluorescent screen which has a suitable positive potential against the specimen. The beam current and the secondary electron current are extremely small, of the order of 10'^^-10'^'* amp. The amplifica- tion of such minute currents with ordinary amplifiers is hope- less, the signal would be drowned by the noise which is mainly caused by the thermal fluctuations of voltage in the input Fig. 36. scanning microscope, experimental form resistor, parallel with the first grid. Let this resistance be R, the fluctuation voltage v, Boltzmann's constant k, the tempera- ture of the resistor T, and the frequency band transmitted by the amplifier AF. Nyquist's well-known formula gives for the mean square voltage fluctuation v^ = AkTR^F (28) The reduces this noise by two ingenious artifices. The first is employing an electron multiplier as the first stage of the
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