Note 1 to entry: For the quantities ${\displaystyle a^{\prime }(t)=\widehat{A^{\prime }}\text{cos}(\omega t+{{\vartheta }^{\prime }}_0)}$ and ${\displaystyle a^{″}(t)=\widehat{A^{″}}\text{cos}(\omega t+{{\vartheta }^{″}}_0)}$, the phase difference is ${\displaystyle \varphi ={{\vartheta }^{″}}_0-{{\vartheta }^{\prime }}_0+2\pi n}$, where n is an integer, chosen so that ${\displaystyle -\pi <\varphi \le \pi }$.