A secondary explosive is less sensitive and is therefore usable in a wider variety of applications. Examples of secondary explosives include TNT and RDX.
Primary explosives
As a very general rule, primary explosives are considered to be those compounds that are more sensitive than PETN. As a practical measure, primary explosives are sufficiently sensitive that they can be reliably initiated with a blow from a hammer; however, PETN can usually be initiated in this manner, so this is only a very broad guideline. Additionally, several compounds, such as nitrogen triiodide, are so sensitive that they cannot even be handled without detonating.
A number of primary explosives are commonly used in blasting caps to translate a signal (electrical, shock, or in the case of laser detonation systems, light) into an action, i.e., an explosion. A small quantity—usually milligrams—is sufficient to initiate a larger charge of explosive that is usually safer to handle.
