[78-L] how long is it?

[Malcolm]

I just got an interesting question from a friend who has nothing to do 
with 78s!
He asked: "If a record didn't go round  and round but a 3 minute 
(groove) went straight,  how (long would the groove be)?"

To which I replied: "There's got to be a math formula for this - how 
does one figure out the length of a decreasing spiral over 2 radial 
dimensions? First, you'd have to know the pitch of the groove - that's 
how many lines per inch along a radius the groove makes. Secondly, you'd 
have to measure the inside and outside radii of the spiral. I think 
that's all that is needed to plug into a set formula.
Someone in 78-land will know this.
I'll ask."

Anyone?

Malcolm