122 ELECTROLYSIS. 



out of the bath and passed through hot sawdust that they can 

 be touched with impunity. By ignoring this direction, it 

 would be utterly impossible to obtain pieces without spots, for 

 the cleanest hand always leaves a slightly greasy spot. 



The pieces are suspended in the bath by means of the 

 copper wire which surrounded them during the operation of 

 cleaning, the anodes are also placed ready in position, and, as 

 we have already said, the current passes at the same moment. 



In order to secure good contacts,* four bars of copper are 

 arranged on the walls of the vats parallel to each other. On 

 two of these bars rest all the rods from which are suspended 

 the pieces to be plated, and which are connected with the 

 positive pole of the source of electricity (Fig. 20). 



Mr. Watt recommends the increase, as much as possible, of 

 the suspension wires when it is proposed to coat lead, tin, 

 and even cast-iron pieces. He also advises that these wires 

 should be cleaned after each operation, giving for this the 

 following reasons : 



After the suspension wires have been used once or twice 

 (and especially when a strong coating has been deposited on 

 the suspended object) the crystalline nickel, deposited on the 

 unprepared surface of the wire, renders the latter very brittle, 

 the deposited metal forming round the wire a tubular envelope 

 which can easily be broken. Wires which have been used a 

 few times for the purpose cease to be suitable, as they grow 

 more brittle and less malleable. 



In order to render them fit to be used again, they must be 

 cleared of the nickel which covers them and annealed; the 

 nickel is got rid of by dipping them in a solution of two parts of 

 nitric acid, one part of sulphuric acid, and four parts of water. 



Some writers advise an energetic electric current at the be- 

 ginning so as to procure a better adherence of the nickel on the 



* We consider it as essential to keep good contacts from the batteries or the 

 machines up to the time pieces are immersed in the bath. The circuit must have 

 no abnormal resistance ; when certain parts of the conductors are not intimately 

 connected together, it is necessary to keep them in a perfect state of cleanliness, 

 and to carefully ascertain that there is no trace of oxide or of dust between the 

 bars, the rods, and the suspension hooks. This precaution is too often neglected, 

 and it is the cause of many failures. 



