https://engineering.stackexchange.com/questions/25482/lowercase-phi-%cf%86-seemingly-used-for-diameter-in-diagram-instead-of-%c3%98
Jan 12, 2019 · On a technical drawing for resistors, the lead diameter was given as $\\varphi 0.55\\text{mm}$. Since they also specify the wire is 24 SWG, and 24 SWG is a diameter