Tke Making of Scales upon Glass or Metal 155 



and boiled linseed oil. After wiping clean, the edges of the 

 slip may be ground and polished. 



(b) A circular scale, upon glass, paper, or metal, is made in a 

 similar manner by the use of a graduating table. This consists 

 of a hardwood base into which a graduated brass circle has 

 been sunk, and a rule one edge of which is coincident with a 

 radius of the graduated circle (see Fig. 115). The paper, 

 glass, or metal is firmly fixed to the board, and the rule 

 applied, the circles or segments drawn, and marks transferred 

 along the radial edge with some suitable instrument a draw- 

 ing pen for paper, a sharp stylus, as previously described, for 

 waxed glass, or a graving tool for metal. The rest of the 

 process is simple, consisting mainly in cleaning up and burn- 

 ishing. Instead of being engraved, brass may be coated with 

 wax, and etched with nitric acid (two parts 1*4 S.G. acid, one 



FIG 115. 



part water) ; cast iron and steel may be similarly treated, but 

 with sulphuric acid, or copper sulphate solution if an actual 

 etching cf the surface is not desired. 



Engraved scales in brass may be silver-plated by suspend- 

 ing them in a solution of silver cyanide and potassium cyanide, 

 connected to the negative pole of a 4- or 6-volt battery, and 

 immersing in the same liquid a silver plate connected to the 

 positive pole of the battery, in the same way as the electro- 

 typing was performed previously. 



The solution is made as follows: 20 grams crystallized 

 silver nitrate are dissolved in 100 c.c. distilled water, to which 

 o'5 c.c. pure nitric acid has been added. A saturated solution 

 of potassium cyanide in distilled water is then prepared, 

 filtered if necessary, the volume determined, and the solution 

 added carefully to the silver nitrate solution until the precipitate 



