# Lowercase phi (φ) seemingly used for diameter in diagram instead of Ø?

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 of 0.559mm, did they simply use φ (phi) when they should have used Ø, or is this actually accepted notation? And if it is accepted usage, does the usage of φ differ from Ø?

• A picture would speak a thousand words.... – GlenH7 Jan 12 at 1:33
• @GlenH7 Wish I had one I could share (NDA). I can tell you the "drawing" in question was nothing more than an orthographic projection of a through-hole (PTH) resistor with "φ0.55mm" in vague proximity to the end of the lead with no other markings. I only figured out it referenced the diameter because elsewhere in the textual spec it said the leads were 24 SWG, which I know has a diameter of 0.559mm. – type_outcast Jan 12 at 13:30

## 1 Answer

As engineers, symbols get re-used due to the fact that there are so many things we need to give a label to, so if someone uses a lower case does it change the thing described?

As long as it is clear what is referred to, then that is what matters. If someone heard verbally " phi " and used lower instead of uppercase...

The table in this link gives the diameter of 24SWG as 0.558mm, see https://www.esr.co.uk/electronics/cable-copper2.htm

• I'm with you on "using what works". In this case, though, there were no other symbols on the drawing. And Ø has no lowercase, nor is it related to phi at all. So are you saying that this usage of phi for diameter is unusual/non-standard? (I've certainly never seen it before, but I'm not an engineer.) It was only because they actually specified the wire gauge three pages later in the spec that I was able to figure out that it was indeed the diameter. So the diagram was either useless, or I'm just not familiar with phi being used for diameter. – type_outcast Jan 12 at 13:33
• It may well be a case of « who » produced the drawing - based on your other comments, but, as you won’t show then all we can do is guess... – Solar Mike Jan 12 at 13:39
• @type_outcast Sure Ø has a lowercase: ø — it's the symbol for diameter ∅ that doesn't have a lowercase. But that is the sort of detail that doesn't really matter. People use "lookalike" symbols all the time, just as you just did. As Mike said, as long as it is clear what is referred to, … – ingenørd Jan 12 at 19:54