The green dots and dashed dark grey where-it's-headed path are for visual reference -- in this case, the ends of the larger upper arc segments are 13 drawing units apart, and the ends of the smaller lower arc segments are 6 units apart.  Snap is turned on at 1 drawing unit.

You could also just draw Arcs and Copy or Array them, then join them into a Polyline.

It's in whatever drawing units are current in your drawing settings, as all entries in the Properties palette always display things.  Just make the start and end tangent vector Y settings equal in value but opposite in sign.  The start-positive and end-negative in mine are appropriate to the direction I drew the Spline, but the opposite would be needed if drawn up-side-down, and if the whole thing runs vertically, use X instead of Y, etc. -- you'll figure it out  It doesn't need to be 1 unit, but that made a good "look" for me at the size of my spiral -- the best appearance could be a very different value depending on the size of things.  You can even play with both X and Y if you don't want the ends springing vertically off the "base" line, to match the fact that in this configuration the Spline doesn't quite cross the base line exactly vertically at intermediate crossings.