Page:Electricity (1912) Kapp.djvu/29

or $D=1cm$. and $F=1$ dyne. The product $f\times Q^{2}$ will then also be unity. All that our experiment tells us is that the product of two things is unity, but it does not tell us the separate value of each of the two things, which is only another way of saying that we do not know and probably shall never know what electricity really is any more than we can know the real nature and value of the ethereal coefficient. We can, however, choose one of the factors, and then the other is also determined. If we adopt the definition of unit mass given on p. 13, then $Q$ is 1 and $Q^{2}$ is also 1. From this it follows that $f$ is also 1, and our general formula simplifies to

$F={\frac {Qq}{D^{2}}}$

In adopting this formula we have arbitrarily settled the magnitude of the unit of electric quantity. It is such a quantity of charge as will give the force of one dyne, if acting on an equal charge at a distance of one cm.

It will be noticed that the train of reasoning followed here is different from that we followed in the case of gravitation. There we started by adopting a particular quantity of ponderable matter as the unit, namely the gram. This is the obvious way, because we know