Flamsteed Astronomy Society

“Webs and bubbles” by David Westcott

— Apr 21, 2008

page 2 of 2

Now for bubbles.   Lin felt she was on solid ground here, so long as we were talking Bollinger.  However, it seems it’s not that simple ...   Move on, gentle reader.

Bubbles are needed in ‘Levels’ used to make sure an instrument is, well, level.  You know — not crooked.  Any old bubble won’t do.  Dave recommends only the finest bubbles from Dom Perignon.  Things are not all they seem to be in precision bubbles ...

 

Builders’ level bubbles are made from best bent glass tubing and filled with light oil

Optical levels and telescope bubbles are made from scientific glass tubing filled with ether.

1.  Shape the glass tube.  The set-up consists of a steel rod bent to the precise shape required, together with a parallel glass tube (L).

Accurate bubbles are shaped by hand, grinding with Carborundum paste

The finished tube is barrel-shaped.

3.  Seal the ends.   One end is drawn into a capillary.  The other end is sealed flat.

 

Ah!   Snag!   How do you fill a tube with ether through a capillary? ...

4.  Use a vacuum!   No Madam, not a Hoover.  Submerge the tubes in ether in a glass bath.  Put it all under a bell jar and pump the air out, which will also remove the air from the tubes.  Release the vacuum and ether will be drawn into the tubes through the capillaries.  Easy!

5.  Adjust the bubble to length.  The tube filled with ether is emptied a little at a time by flicking with the finger until the length is correct.

It is then sealed VERY CAREFULLY by passing the end very quickly through the flame of a Bunsen burner ... (don’t try this at home).

Done!

 

Now open the Dom Perignon.

 

Thanks Dave!  Quite sparkling. 

 

Effervescent even.

2.  Test the bubble.  The test rig is a master tube on a hinged plate with a micrometer screw at one end. 

The ground tube is corked and filled with ether.

It is compared with the master (very fiddly) and marked +/- to show the correction needed

6.  Etch the bubble.  The bubble is coated in acid-resistant wax and divisions marked through the wax using a dividing engine.  The bubble is then exposed to hydrofluoric acid fumes in a fume cupboard.  The engraved divisions are filled with black wax.

7.  Finally ...  the bubble is re-tested as before, and mounted in its metal mount.