Page:Elektrische und Optische Erscheinungen (Lorentz) 013.jpg

 will be given only by expression (2), when we don't apply it to the mentioned sphere, but to an arbitrary formed larger space.

Of course $$\bar{\phi}=\phi$$ everywhere, as soon as $$\phi$$ doesn't show rapid changes.

Furthermore we easily find

m. By the average of a vector $$\mathfrak{A}$$ we understand a vector — it may be called $$\bar{\mathfrak{A}}$$ —, whose components are the averages of $$\mathfrak{A}_{x}, \mathfrak{A}_{y}, \mathfrak{A}_{z}$$. Consequently we have