NICKEL-PLATING . 127 



when the bath is cold, and this is particularly advantageous 

 when the two hands are occupied. 



The nickel-plating of these small objects can even be effected 

 without the help of an external current by using a slightly acid 

 bath and placing the objects to be coated in contact with spirals 

 of zinc. It is then sufficient to keep the bath in a state of 

 ebullition for a short time, stirring it with a zinc rod. 



The duration of the immersion varies according to the thick- 

 ness of coating which it is desired to obtain ; for ordinary articles 

 not liable to be damaged by friction, fifteen minutes will be 

 sufficient; for ironmongery articles of current manipulation, 

 half-an-hour is required ; and for nicely finished articles an 

 immersion of one hour seems to us to be a maximum when a 

 Gramme machine is used. With batteries the pieces should 

 remain two hours in the bath when an average coating is re- 

 quired, and five hours for a thick coating. 



ELECTKOMOTIVE FOKCE OF THE CUKRENT. When nickel- 

 plating was effected by means of batteries, the platers, accus- 

 tomed to low-resistance baths, always joined their cells in 

 series, and could not obtain the true colour of nickel. The 

 deposit was of a pale yellow hue instead of having the silvery 

 whiteness which it now possesses. 



Experience shows that a good dynamo machine for nickel- 

 plating purposes must have an electromotive force capable of 

 varying between 1 and 8 volts. Mr. Sprague, who should 

 always be consulted on matters of electric intensity or electro- 

 motive force, recommends beginning with 5 volts and finishing 

 with about one. We consider it useful to reproduce the reasons 

 which he gives for it : " The difficulty of nickel-plating does 

 not consist in the choice of the solution but in the direction of 

 the operation, for nickel is different from other metals in this 

 respect, that the deposition is always accompanied by a con- 

 siderable development of hydrogen, resulting in a loss of force ; 

 the object is therefore to obtain the least possible quantity of 

 gas and the greatest possible quantity of nickel. Another 

 consequence is that the deposition being apt to contain the 

 gases, may become porous or scaly, in which case the coating 

 has a tendency, as soon as it has reached a certain moderate 



