The present paper describes an investigation of the physical factors affecting the ignition of solid explosives by heated filaments embedded in the medium. The filaments were composed of fine resistance wire and were heated electrically, the critical thermal energy required to cause ignition being measured for wires of different geometrical, thermal and electrical characteristics and for different times of heating. Systematic variation of these factors enabled the energy equation for the ignition process to be formulated and its terms analyzed, the technique involving extrapolation to zero time of heating as a means of eliminating heat losses from the ignition system, and extrapolation to zero diameter of wire in order to eliminate terms involving the heating element; the former simulates the ideal case of a heat-insulated ignition system and the latter that of a line source of heat. The energy equation for ignition in these circumstances takes a simple form which implies that, at the moment of ignition, the heat supplied to the ignition system always equals the heat gained by the system plus the heat lost, the absence of any term representing heat generated by chemical action being very significant. For a given ignition system, the amount of heat absorbed up to the moment of ignition is shown to be independent of time, so that the increase in ignition energy with increasing time of ignition is wholly attributable to the heat losses sustained by the ignition system during the heating process. Further analysis shows that the critical factor governing ignition in systems of the type considered is the temperature, and that the geometry of the heating element probably determines the amount of explosive which must be raised to the critical temperature to ensure ignition.
The ignition of solid explosive media by hot wires