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244 F I R E R E S E A R C H The discharges between electrodes of certain low-boiling-point metals are known to be capable of igmting flammable gases at much lower levels of arc energy than wi th other metals, and i t has been observed that during the course of the discharge these low-boihng-pomt metals emit relatively large amounts of metal vapor and metal particles This current investigation was carried out in order to determine whether or not the small quantities of metal vapor or particles which are ejected into the surrounding gas during a discharge are capable of causing an igmtion I n one group of experiments, metal wires were exploded by discharging a capacitor bank through them and some of the explosion products were allowed to pass through a small hole into a chamber contaimng a flammable mixture of methane and air The results showed that all but the metals w i th the lower boihng pomts readily caused ignitions Schlieren photographs of the development of the igmtions indicated that, whereas individual particles of aluminum readily caused igmtions, individual particles of the other metals examined (Cd, Cu, Fe, Pb, Sn, W, and Zn) did not I n another group of experiments, small quantities of vapor and particles were produced by focussing the output of a ruby laser onto the flat surface of samples of both metals and nonmetals The amount of laser energy necessary to produce sufficient vapor to igmte a number of flammable atmospheres was determined for these matenals, and i t was found that this amount increased considerably as the boiling point of the material decreased I t was concluded that the ease with which arc discharges between low-boihng-point metals cause igmtions was not due to the ignition of the flammable atmosphere by the metal vapor and particles emitted during the discharge Weinberg, F . J . and Wilson, J . R . (Department of Chemical Engmeenng, I m - perial College, London, England) " A Prehminary Investigation of the Use of Focused Laser Beams for Mimmum Igmtion Energy Studies," Proc Roy Sac, Ser A 321,41 (1971) Sections: B, I Subjects: Lasers, Ignition, Minimum igmtion energy; Plasma Authors' Abstract The plasma produced by focusing a passively Q-switched 30 J ruby laser beam is studied as regards its suitabihty for minimum igmtion energy measurement B y comparison wi th electric spark discharges, i t would appear to offer shorter times and smaller volumes of the imtiating plasma, as well as freedom from energy losses to electrodes and elsewhere within the circuit The growth of the mitatmg plasma as well as that of the flame kernel are studied as a function of pressure and com- position, the former using an optical delay line with a Schlieren system and the
