Its principal commercial source is the **fraction** of coal-tar which distils between i 50 and 200° C., in which it was discovered in 1834 by F.

Very convenient and accurate instruments based on the above principles have been devised by Lord Kelvin, and a large variety of these ampere balances, as they are called, suitable for measuring currents from a **fraction** of an ampere up to many thousands of amperes, have been constructed by that illustrious inventor.

K= (I +2a)/(I -a), or a=(K-I)/(K+2), where K is the dielectric constant and a the **fraction** of the total volume actually occupied by matter.

We can so determine these n covariants that every other covariant is expressed in terms of them by a **fraction** whose denominator is a power of the binary form.

With wireless telegraph transmitters, in which the oscillatory discharge of a condenser is used to create oscillations in an antenna, labours under the disadvantage that the time occupied by the oscillations is a very small **fraction** of the total time of actuation.