# Page:Thomson1881.djvu/2

 ${\displaystyle f=-{\frac {e}{4\pi }}{\frac {d}{dx}}{\frac {1}{\rho }}}$, ${\displaystyle g=-{\frac {e}{4\pi }}{\frac {d}{dy}}{\frac {1}{\rho }}}$, ${\displaystyle h=-{\frac {e}{4\pi }}{\frac {d}{dz}}{\frac {1}{\rho }}}$;
 ${\displaystyle {\frac {df}{dt}}=-{\frac {ep}{4\pi }}{\frac {d^{2}}{dx\ d\xi }}{\frac {1}{\rho }}}$, ${\displaystyle {\frac {dg}{dt}}=-{\frac {ep}{4\pi }}{\frac {d^{2}}{d\xi \ dy}}{\frac {1}{\rho }}}$, ${\displaystyle {\frac {dh}{dt}}=-{\frac {e}{4\pi }}{\frac {d^{2}}{d\xi \ dz}}{\frac {1}{\rho }}}$;