Page:Elektrische und Optische Erscheinungen (Lorentz) 114.jpg

 Second, as regards the work, we don't have to care about the last member in equation (15) and the analogous formulas. Only the "tensions" come into account, and

is the work of the tension with respect to $$d\sigma$$. The components of this tension are

from which it follows

Eventually, A means the sum of the two last members in (107).

The given values now actually satisfy the condition (109).