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.
Overview of the circuit with the 555 in the middle and the potentiometer on the left.