Wehnelt

Burr, L.E. (1997) Reptile enrichment scenting for a response. Animal Keepers Forum. 24,122-23. Burrell, K., Wehnelt, S. and Rowland, H. (2004) The effect of whole carcass feeding and novel scent on jaguars (Panthera onca) at Chester Zoo. British and Irish Association of Zoos and Aquariums Zoo Federation Research Newsletter 5, 4. 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have a repetition frequency... 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have repetition frequency about 800 Hz. They are used to deflect the electron beam horizontally and vertically over the exit window plane. The scanner can be equipped by two cathodes for maximum output. Then, the width of the exit window is more than double that of a standard unit with a single cathode. The exit window containing the 12-15 prn-thick titanium foil is relatively large to assure an effective cooling of the foil. 

Mg (g). Wehnelt and Musceleanu1 measured directly the heat of vaporization of magnesium. Vapor pressure data were obtained by Ditte,7 Greenwood,8, 5 von Wartenberg,4 Ruff and Hartmann,1 and Hartmann and Schneider.1 The values of the energy states of gaseous monatomic magnesium are from Fowler,8 Turner,1 and Soderqvist.1... 

Mg (liq.). Data on the heat of fusion of magnesium were reported by van Aubel,1 Wehnelt and Musceleanu,1 Awbery and Griffiths,1 and Zalesenski and Zulenski.1... 

The design of a SEM is shown in Fig. 2. It consists of the electronic gun (1) the Wehnelt cylinder (2) the anode (3) and beam alignment coils (4) on the top of the instrument. The condenser lenses (5) the aperture, and the objective lens (6) focus the beam onto the specimen that is mounted on the specimen holder (7). The latter one could be moved in X-, Y-, and Z-direc-tion within the specimen chamber. In addition, the sample could be moved by rotation. The arrangement to create the electronic beam is shown in Fig. 3. 

Fig. 2 The design of a secondary electron microscope where 1, electronic gun 2, Wehnelt cylinder 3, anode 4, beam alignment coils 5, condenser lenses 6, objective lens 7, specimen holder. (From Ref. " l)...

The general structure of an electron gun is composed of three main parts cathode or electron source, Wehnelt electrode and anode, as illustrated in Figure 3.2. Electrons are emitted from the surface of the cathode and accelerated by an electric field toward the cathode. The Wehnelt electrode is placed between the cathode and the anode. It is biased a few hundred volts negative with respect to the cathode in order to stabilize the electron beam against voltage fluctuation by... 

Focusing of the electron beam use of a Wehnelt or electromagnetic lenses... 

The publication of Wehnelt s paper on his current-interrupter in 1899 provided a new turn to the research on electrochemical discharges. For the first time a technological application was developed based on the electrochemical discharge phenomenon. In the same year, Wehnelt s invention was commercialised... 

Figure 2.2 Experimental apparatus used by Wehnelt for his studies on electrochemical discharges [119]. After a first series of experiments (left), he improved the set-up by enclosing the active electrode c in a glass tube d (right). 

Wehnelt s current-interrupter became very popular and a competitor to the Ruhmkorff coil, a device that most of us still know from physics lessons... 

Figure 2.4 The Wehnelt interrupter commercialised in 1899 by the German company Ferdinand Ernecke (left). The length of the platinum electrode e could be adjusted. The electrical contact was made with the help of the cooper rod / [119]. On the right is shown a model commercialised in France by Armagnat-Carpentier [10].

In the same year as Wehnelt s first publication on his current-interrupter, E.W. Caldwell [17] and H.Th. Simon invented independenlty a different version of the electrolytic interrupter. Their interrupter used a small hole drilled in a wall separating the cathodic and anodic compartments of an electrolysis cell. As in the Wehnelt interrupter, the periodic formation of a gas film regularly interrupts the current. 

Figure 2.5 Model of the Wehnelt interrupter. The switch is closed as long as no electrochemical discharges take place. As soon as the gas film is built, the switch is opened.

Considering the importance of this first technological application of electrochemical discharges, we will present here a simplified theory of Wehnelt s current-interrupter. A resistance R and inductance L is connected to the Wehnelt interrupter. Based on early mathematical models of the device [83,99], the Wehnelt interrupter is considered to be a resistance Rv in series with an electrical switch (Fig. 2.5). 

Once the device is connected to a constant voltage U, the current in the Wehnelt interrupter grows (the switch in the equivalent circuit is closed). An increasing volume of gas is produced until a gas film is built, isolating the active electrode from the electrolyte. The switch in the equivalent circuit is now open. The unstable gas film is quickly removed from the active electrode and the process starts again. From the equivalent circuit, the evolution of the current is given by ... 

Equation (2.3) is able to explain the operation of the Wehnelt interrupter. The number of interruptions is inversely proportional to the inductance L. As Rk= R + Rv and Rvactive electrode length (cylindrical geometry), it follows that n will increase inversely proportional to l, as first experimentally observed by Ludewig [83]. 

By introducing the normalised voltage U = U/ Um a quantity that is important for further theoretical description of electrochemical discharges, one obtains a particularly elegant relation for the Wehnelt interrupter (Fig. 2.6) ... 

Figure 2.6 Number of interruptions n in the Wehnelt interrupter as a function of the voltage U, as predicted by Equation (2.3).

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