Page:Electricity (1912) Kapp.djvu/155

$2\pi R$ cm. length, carrying a current of $J$ units in a field of induction $B$. Generally, writing $l$ for the length of the wire in cm., and expressing current strength in amperes, we have the force in dynes

$F={\frac {J}{10}}lB$

Let us apply this to the wire on the armature of a dynamo machine. Suppose the armature is 10", or 25.4 cm. long, and the induction is 8000. With a current of 50 amperes through the wire we have a tangential force of

$F={\frac {50}{10}}25.4,8000$ dynes;

or,

$F=1.03$ Kg., or $F=2{\frac {1}{3}}$ lb.

Since on the circumference of an armature there may be hundreds of such wires, the majority of them simultaneously under the same influence, it is easy to understand that the combined tangential force, which in the case of an electric generator has to be overcome by the force of the driving engine, and in the case of an electromotor produces motion, may become very considerable.

In deducing the formula for the force