Glass Working -Exercises 113 



mercury. In this case, the copper and platinum wires are first joined 

 (soft solder being satisfactory for this purpose), the platinum foil 

 welded to the platinum wire, a nick made with cutting pliers upon the 

 copper wire where it is intended ultimately to be broken, and the 

 whole inserted into a partially closed straight glass tube, which is 

 then shrunk upon the platinum wire carefully, in order to avoid 

 melting the soldered junction. The copper wire is then broken 

 at the nick by a slight twist, pull, and rocking motion, such as 

 the tube will permit, and finally the tube bent into shape and 

 bordered. This method has the advantage that a convenient 

 junction for bare wires is made without the fear of breaking off a 

 delicate platinum loop either during an experiment, or in cleaning, 

 while it needs much less platinum to make a satisfactory junction. 

 Where the saving of platinum wire is not all-important, the electrode 

 may be constructed as shown in Fig. 105 (B). 



To increase the surface of the electrode, the platinum foil may 

 either be wrapped into a hollow cylinder around a 

 glass tube (Fig. 105 (B) ), or be crinkled by being 

 bent backwards and forwards over the edge of FlG> Io6 



a piece of glass plate, and subsequently pulled 

 out a little in the shape indicated in Fig. 106, either being performed 

 after welding on the connecting wire. 



EXERCISE 18. To make a platinum-tipped jet. 



This is a simple exercise, without which, however, this section 

 would be incomplete. 



Cut a piece of platinum foil 2 cm. x I cm. and roll it round a 

 stout pin, or knitting-needle, so producing a tube i cm. long. This 

 may be heated while still on the needle, in the blow- 

 pipe flame, and welded together by smart light blows fl 

 as already described (see p. 61), though this welding (f\j 

 is not really necessary. 



A piece of glass tubing 5-3 mm. diameter is then 

 constricted somewhat at one end by drawing out 

 while thickening, as in Exercise i, and the tube 

 cut off where the diameter of the bore will just permit 

 the introduction of the platinum tube. The needle Fl 9* 107. Pla- 

 is withdrawn from the platinum tube, which is half j^um-tipped 

 inserted in the glass one, and by carefully heating 

 the glass round the inserted portion it may be shrunk evenly 

 upon the platinum, producing a jet similar to that in Fig. 107. 



I 



