Figure 3.2 shows how the output varies with time; it is an exponential
decay. The output rises to its maximum value at time t = 0 and then after
13 it drops to 0.37 of the initial value, after 2z it is 0.14 of the initial
value and after 33 it is 0.05 of the initial value. Thus by about a time
equal to four times the time constant the output is effectively zero. The
exponential term tends to a zero value as the time t tends to an infinite
value.