This experiment (and the next experiments) are centered around the 555 timer chip. The circuit that been built (figure 4.14 of Make:Electronics) results in the monostable mode of the 555 chip. Once triggered the 555 emits a single pulse of a fixed duration. I lacked the required 5K linear potentiometer and I used a 2.5K instead. I decreased the resistance of the potentiometer step by step and at a certain threshold the led that is connected to the pin 3 (output) emitted a pulse. At this time I measured a voltage on pin 2 (trigger) of 3V. This experiment is straightforward and I encountered no problem. Afterwards I changed the capacitor on pin 6 (the threshold pin) from 47uF to 22uF. The duration of the pulse halved as expected.