What is the tachometer princibal?

THERE ARE TWO TYPES OF TACHOMETER. 1.D.C. TACHOMETER 2.A.C. TACHOMETER ALSO DIGITAL TACHOMETERS CAN BE CONSTRUCTED using various principles..... D.C. TACHOMETER its construction is very similar to a D.c. generator. it works on the principle of generation of emf wen a rotating conductor cuts the magnetic flux......when magnetic flux is constant then the emf produce is directly proportional to the speed of the armature that is coupled to the shaft of motor. A.C. tachometer its construction is similar to a two phase servo motor.... and works conversly of it....