# If electrons are too small to be weighed with a pair of scales, how do we know their mass so precisely?

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If electrons are too small to be weighed with a pair of scales, how do we know their mass so precisely?

In: Physics

A scale measures the interaction of the item with earth’s gravity. Which is fine for these kinds of items but for electrons you for example can measure their interactions with light or something similar to determine their weight.

There were two different experiments:

1. J. J. Thompson’s experiment to measure the ratio of an electron’s mass to its charge.
2. Millikan’s Oil Drop experiment, which measured an electron’s charge.

With both of these numbers you can calculate the mass of an electron.

The oil drop experiment measured something very tiny, an electron’s charge. It did this by adjusting an electric field so that tiny charged drops of oil stayed suspended under gravity. By taking lots of observations they saw (roughly) that these droplets fell only at specific speeds that were multiples of some other number – and from there they could compute the charge of an electron.