You can't just digitise it, in the same way you can't just make your standard kg, drop it on to a normal set of scales, go "It's one kg". It would either be circular and useless (one kg is equal to one kg... so what?) or dependent on the scale (one kg is the mass required to deflect this particular scale's spring 0.05mm at a certain location on earth under certain conditions). Digital scales just measure the same deflection essentially by measuring the change in electrical resistance on a piece of wire due to stretching, by reading the voltage, and quantising it to a finite number of bytes.
The only way you can get beyond an arbitrary prototype is by defining it in terms of some physical constant (and even then, except for "natural units", the coefficient we apply is arbitrary to get something of the scale we want). For example, we define the meter by the distance light in a vacuum travels in a certain fraction of a second. The speed of light in a vacuum depends only on the electromagnetic permeability/permittivity of a vacuum, which is a universal constant. However the time we use is arbitrary, and is just so that the 'meter' we get from this is close to the historical meter. The second in turn is defined by the radioactive decay of cesium 133, which is like a natural clock that will be the same everywhere.
This post was edited by TIMMY213 on Jan 4 2014 10:26pm