This is also called Reaction Time Apparatus. It consists of two parts : 1. Reaction time 2. Chronoscope, The reaction time apparatus is a wooden board with screen in the middle. On one side of the screen (subject's side) there are four different colored lights and a press key. The key is connected to the lights. If the key is pressed while the light is on, the circuit will break and the light will go off. The other side (experimenter side) has key which serves to switch on the lights on the subject side. Along with this a buzzer is also fixed on the experimenter side. The experimenter can switch on or switch off the lights or ring the buzzer. Chronoscope is an electrically operated timing instrument which measures very short intervals of time. Whenever the experimenter switches on the lights or the buzzer, the chronoscope starts working and when the subject responds, the chronoscope stops. The reading of the chronoscope gives the reaction time.