132 Laboratory Arts 



copper strip or a copper wire, wrapped around several times, 

 and the whole " sweated " J together. 



A good exercise is the manufacture of a constant E.M.F. 

 (ferric chloride) cell. 



A piece of arc lamp carbon 12^5 cm. long is drilled along 

 its axis for i cm., and at an angle of about 1 10 to its axis at the 

 end of this hole. The carbon is then connected to the nega- 

 tive pole of a cell, and immersed in a copper-depositing cell to 

 a depth of 2 cm., so that the whole of the drilled-out passages 

 are coated with copper. The deposition being completed, the 

 carbon is removed, cleaned and dried. 



A freshly scraped No. 20 copper wire is pushed down the 

 axial hole, along one of the side holes, and pulled through for 

 about 15 to 20 cms. This is then wrapped round the carbon 

 several times, and the free end finally inserted into the second 

 side hole, making a neat and secure job. The junction is then 

 well soldered, the hole through which the wire passes being 

 completely filled up with solder. 



This carbon is secured at a distance of about 2 mm. to a 

 piece of hoop iron 12*5 X 2*5 cm. interposing rubber or wood 

 of the required size, and binding the two pieces together with 

 twine. A second piece of No. 20 copper wire is soldered to 

 the hoop iron, and the resulting element, immersed in a 10 

 per cent, solution of ferric chloride, will give a constant E.M.F. 

 of from 075 to 1*0 volt. 



Quartz fibres (or glass fibres, which form a reasonable 

 substitute in many rough forms of apparatus) may be similarly 

 treated. They are coated with blacklead by rubbing between 

 finger and thumb, immersed in a plating bath, as above, and 

 copper deposited for about an inch from the end. This is 

 then dried, dipped in zinc chloride solution, and stroked 

 through a bead of solder upon a hot soldering iron. The 

 support is tinned and the fibre soldered in place without 

 trouble. 



Unnecessary copper may be removed with a piece of cotton- 

 wool dipped in nitric acid. 



1 I.e. heated before the blowpipe in presence of ZnCl 2 and soft solder. 



