The purpose of a detonator is to impart to the explosive, immediately surrounding it, an initial impulse of sufficient strength to set up an explosive wave, causing propagation at high efficiency through the entire explosive charge, which, due to the sudden application of intense heat and shock from the detonator, transforms the explosive from a solid to a gaseous state. PLAIN DETONATORS. The different grades of detonators are designated by the weight of charge contained in them; the sizes used commercially are Nos. 6, 7 and 8. Beneficial results have been obtained from experimental tests carried out during the past few years in an endeavour to find a detonating mixture to give increased efficiency and to impart a more intensified impulse per given volume than the fulminate of mercury potassium chlorate composition - hence the nitro-substitution mixtures which have proved to increase initial impulse.