Page:Electricity (1912) Kapp.djvu/23

Rh unit for the force, but derive it from the three fundamental units of mass, length and time, since every physical quantity may be expressed by reference to these three units. If we choose the centimetre as the unit of length, the gram as the unit of mass and the second as the unit of time, we adopt what physicists call the centimetre-gram-second system of measurement. For this the abbreviated designation c.g.s. is customary. In this system force is a so-called derived unit, namely that force which, acting steadily in the same direction for a second on the mass of one gram, will give it an acceleration of one cm. per second per second. This unit is called the dyne, and from what has been said above it is obvious that 981 dynes go to one gram, or 981,000 dynes (approximately one million dynes) are equivalent to the kg. If we now repeat the calculation, using the c.g.s. system, we get the force in dynes if we express in the general formula

$$F = f \frac{Mm}{D^2}$$

the masses in grams and the distance in cm. The ethereal coefficient then has the value

$$f = 6.6 \times 10^{-8}$$