# Why Coulomb chose 1.6x10^-19

1. Mar 7, 2012

### kjamha

Does anyone know why Coulomb chose 1.6x10^-19 for the charge of an electron? (or why he chose 6.25x10^18 electrons for 1 coulomb)

2. Mar 7, 2012

### FroChro

Re: Why 1.6 x 10^-19?

I think your question is ill-posed. I believe that in times of Coulomb they hadn't known that there is some ''atom'' of charge, much less what is its magnitude.

3. Mar 7, 2012

### Staff: Mentor

Re: Why 1.6 x 10^-19?

Coulomb didn't choose anything. Unit of charge (defined as charge transferred by 1 ampere current in 1 second) was named Coulomb in 1881, and Coulomb died in 1806. At the time Coulomb (unit of charge) was defined, electron was yet to be discovered.

4. Mar 7, 2012

### kjamha

Re: Why 1.6 x 10^-19?

Thanks for the history!

But isn't 1 Ampere equal to 1 coulomb per second - so are you saying 1 coulomb per sec in one sec? Can you clarify this for me?

5. Mar 7, 2012

### cepheid

Staff Emeritus
Re: Why 1.6 x 10^-19?

You are concerned, because the definition seems circular (a coulomb is the charge transferred by a current of one ampere in one second, and an ampere is the current produced when one coulomb of charge flows past in one second).

But the definition is not circular, because there is a separate and independent definition of what an ampere is. See my post in this thread:

EDIT: Why don't I just quote my post from that thread here:

6. Mar 7, 2012

### Dickfore

Re: Why 1.6 x 10^-19?

The real question is why the fine structure constant:
$$\alpha \equiv \frac{k \, e^2}{\hbar \, c} = \frac{1}{137.0}$$
where k is Coulomb's constant has the numerical value that it does.

7. Mar 8, 2012

### kjamha

Re: Why 1.6 x 10^-19?

Thanks Cepheid, but now I am wondering why they chose 2e-7 N - seems odd.

8. Mar 8, 2012

### Staff: Mentor

To quote wikipedia: