Lab 07 — Experiment

Millikan's Oil-Drop Experiment

In 1909 Millikan suspended charged oil droplets between capacitor plates and measured the elementary charge e. The droplet here has a known mass and a negative charge — some hidden multiple of e. Raising the voltage pulls it upward, so hunt for the voltage where the electric force exactly balances gravity. When it hovers, hit “Record”. Repeat over many droplets and the measured charges pile up only at integer multiples of e: charge comes in lumps, and you get to rediscover it.

qE = \dfrac{qV}{d} = mg \quad\Rightarrow\quad q = \dfrac{mgd}{V}, \qquad q = n\cdot e, \quad e = 1.602\times 10^{-19}\ \mathrm{C}

The droplet turns green when balanced. Arrow keys fine-tune the voltage

VoltageV = 0 V

Droplet motion↓ fallingRecords0

The recorded charges q = mgd/V land only on the 1e, 2e, 3e… lines — that is charge quantization. Millikan himself measured e = 1.59×10⁻¹⁹ C — slightly off because the accepted value for the viscosity of air was itself in error — and won the 1923 Nobel Prize. Here the droplet mass is given; in the real experiment it is deduced from the fall speed with the field off.