State the meaning of quantisation of charge.
A student repeats an experiment to find the elementary charge. A charged oil drop is placed between two horizontal metal plates, as shown in Fig. 8.1. The plates are separated by a distance of $7.0\,\text{mm}$. The lower plate is earthed. The potential of the upper plate is raised gradually until the drop remains stationary. The potential needed for the drop to stay stationary is $680\,\text{V}$. The weight of the oil drop, after allowing for the upthrust of the air, is $4.8 \times 10^{-14}\,\text{N}$. Calculate the charge on the oil drop.
The student carries out the experiment again and obtains the following charges on the oil drops: $3.3 \times 10^{-19}\,\text{C}$, $4.9 \times 10^{-19}\,\text{C}$, $9.7 \times 10^{-19}\,\text{C}$, $3.4 \times 10^{-19}\,\text{C}$. Use these values to suggest a value for the elementary charge. Explain your working.