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Minor fluctuations of velocity are frequently observed in gaseous mixtures whose compositions lie near to the limiting values of sustained propagation of detonation. Various possible origins of these fluctuations are discussed. In cylindrical tubes, they commonly take the form of a spinning detonation front. In order to study these effects in detail, previously described methods for making accurate observations near the limits have been extended in various ways. Use of multiple probes set radially in a standard detonation assembly permitted determination of the profiles of spinning ionization fronts to the detonation wave. In some cases, the existence of multiple spin heads detected by these means could be confirmed by soot-trace techniques. For any given mixture, fluctuations in the form of spinning ionization fronts are often remarkably reproducible in a particular tube.

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COXTESTS Page 9 1. Introduction 298 Q 2. T h e main characteristics of oxide cathodes 301 2.1. Activation and de-activation 301 2.11. Activation 301 2.12. De-activation 304 305 2.21. Time changes on drawing of current 305 2.22. Life failure 307 2.23. Temperature effects 308 2.3. Lack of saturation 308 309 3.1. Introduction 309 3.2. Conduction 310 3.21. Experimental data 310 3.22. Theory of the conduction process 312 3.3. Contact phenomena and the core coating interface 317 3.31. Contact phenomena 317 3.32. RGle of the core metal and the interface 318 3.4. Diffusion 319 3.5. Evaporation 321 3 22 4.1, Introduction 322 4.2. T h e Wilson model 322 4.3. de Boer's model 326 4.4. T h e model of Uehara and Takahari 327 4.5, General considerations 328 330 5.1. Luminescent properties 331 5.2. Thermionic emission under pulsed conditions 331 5.3. Secondary electron emission under electron bombardment 335 3 6. Conclusions 336 338 References 339 2.2. Time changes exhibited by the fully activated cathode8 3. Processes determining the emission $ 4. General theories of emission Q 5 . Other phenomena occurring with oxide coated cathodes Appendix : List of recurrent symbols used § 1. I N T R O D U C T I O NT H E emission of charged particles, and in particular of electrons, from hot bodies is a well-known phenomenon. It is due to thermal excitation of the electrons within the body to a state of sufficiently high energy to overcome the energy-barrier set up by image forces and polarization of the atoms at the surface. If the hot body is surrounded by a vacuum and no external field is applied, a negative space-charge is set up round the emitter and most Estate, London.