Simple Apparatus (as applied to intrinsic safety) :
A device that will not generate nor store more than 1.2V, 100mA, 25mW, or 200J. Examples are switches, thermocouples, light-emitting diodes, and resistance temperature devices
EESiFlo North America, Terminology and Glossary: Explosion Proof & Hazardous Areas, Pennsylvania, USA 2002, flowmeterdirectory.com/flowmeter_terminology, (Mrtm/ Explo Proof and Haz Areas - Amer) {561}