The triggering processes of some types of Schmitt circuit, have been investigated. Their switching speeds have been calculated and checked by experiments. The main results are as following:(l)The switching speed of an ordinary d. e. coupled Schmitt circuit is rather slow.(2)The use of break-away diode in the Schrnitt circuit) shortens the triggering process considerably.(8)With other conditions unchanged, the switching spaed of an a. c. coupled Schmitt circuit is 3 times faster than a d. c. coupled Schmitt circuit. Combined use of (2) and (3) will greatly increase the switching speed of the circuit,hence this circuit is particularly suited for fast counting work.The accuracy of these circuits in measuring the amplitude of short pulses is also discussed in the paper.