ELECTRO-PLATING 



2857 



ELECTRO-PLATING 



from the resin it will give a spark 

 if the knuckle or any conductor be 

 brought near it. 



The sole-plate performs an im- 

 portant function by the mutual in- 

 duction which takes place between 

 it and the upper plate or cover. 

 When the latter develops its posi- 

 tive charge on being put in contact 

 with the earth, the sole-plate re- 

 ceives a corresponding negative 

 charge from the earth, and in this 

 way the original positive charge of 

 the cover due to the negative 

 charge of the resin base may be- 

 come an appreciable amount. The 

 electrophorus may be worked al- 

 most indefinitely, i.e. every time 

 the cover is put back on the base 

 and its surface touched with the 

 finger the action takes place and a 

 spark may be obtained when the 

 cover is lifted. 



An arrangement has been de- 

 vised by which the connecting of 

 the cover with the earth is per- 

 formed automatically. In this form 

 of the instrument the cover is 

 fitted with a strip of tinfoil which 

 makes contact with the sole-plate 

 when the cover is laid on the resin, 

 so amounting to the same thing as 

 connecting the cover with the 

 earth through the finger. Nothing 

 is created by the action of the elec- 

 trophorus, although something 

 appears to be. The initial charge of 

 electricity is due to the mechanical 

 energy expended in rubbing the 

 resin, supplemented by the me- 

 chanical energy expended in lifting 

 the cover. The influence electrical 

 machine may be regarded as a 

 mechanical electrophorus acting on 

 the same principle. 



Electro-plating. The deposi- 

 tion of a metal on another sub- 

 stance, usually another metal, by 

 electro-chemical action, either for 

 the purpose of protecting the 

 latter metal from corrosion, as 

 when iron is electro-plated with 

 copper, or for the purpose of giving 

 to a comparatively cheap metal the 

 appearance and some of the pro- 

 perties of one more costly, as when 

 a teapot of Britannia metal is 

 plated with silver. The art is based 

 upon the discoveries or inventions 

 of Volta and Galvani in connexion 

 with electro-chemical action at the 

 end of the 18th century. The first 

 application of those discoveries to 

 plating appears to have been made 

 by Jacobi, at St. Petersburg, who, 

 in 1838, published a description of 

 his process of reproducing line en- 

 gravings on copper by galvanic 

 action. A similar application was 

 made about the same time in Great 

 Britain by .Thomas Spencer, of 

 Liverpool, by whom shortly after- 

 wards the first electro-plating busi- 

 ness was started. 



The scientific principle upon 

 which the art is immediately based 

 is described in the article on elec- 

 trolysis. The operation can be 

 carried out on a domestic scale with 

 small and simple apparatus ; but 

 when carried out on a commercial 

 scale a large vat or bath is used, 

 constructed usually of stout wood 

 lined with lead or slate, though 

 sometimes asphalt or cement is 

 used, or the vat may be built of 

 enamelled iron. It is usually rect- 

 angular in shape, and is fitted with 

 a flange round the top, to which 

 are attached two rectangles made 

 of brass tubing, one being a little 

 larger all round and fixed a little 

 higher than the other, as shown in 

 the illustration. The rectangles, 

 and the vat itself, are insulated 

 both from one another and from 

 the earth. 



The outer ring is used to take the 

 current into the vat, and is there- 

 fore known as the anode ring ; the 

 current passes out through the 

 other rectangle, the cathode ring. 

 The bath is filled with a solution 

 which varies according to the na- 

 ture of the work to be done. The 

 electric current is furnished either 

 by a dynamo or by an electric 

 battery. The articles to be plated, 

 when of convenient size, are sus- 

 pended in the solution by means of 

 wires from crossbars resting on the 

 cathode ring ; while plates of the 

 metal which is to be deposited on 

 the articles are suspended from 

 similar bars resting on the outer 

 or anode ring. The arrangement 

 enables a considerable number of 

 articles to be placed in one vat 

 together with an appropriate num- 

 ber of anode plates, which may be 

 disposed along the brass rectangle 

 as most convenient. 



A simple example will serve to 

 illustrate the entire process, what- 

 ever the metal that is to be de- 

 posited. Let it be supposed that 

 a teapot of pewter or other alloy is 

 to be plated with 

 silver. The teapot is 

 first made as nearly 

 as possible chemi- 

 cally clean, 

 which is done 



Electro-plating. Bath showing how articles 

 are electro-plated with silver 



by first boiling it in an alkaline solu- 

 tion to remove grease, washing 

 freely in water, dipping in a nitric 

 acid solution, washing again, after 

 which it is " quickened "" by dip- 

 ping in a solution of nitrate of mer- 

 cury in order to deposit a film of 

 mercury on the metal, which assists 

 the deposition of the silver. The 

 wire to suspend the teapot in the 

 bath is attached immediately after 

 the dipping in the mercury solu- 

 tion in order that the teapot need 

 not be again touched with the 

 hands. The teapot is suspended in 

 the vat from one of the crossbars 

 of the cathode ring, as shown in the 

 figure. The solution is prepared 

 from cyanide of potassium and 

 cyanide of silver precipitated from 

 nitrate of silver, in water. It con- 

 tains 1 oz. of silver to the gallon. 



The anode plate is of commerci- 

 ally pure silver ; it is, of course, con- 

 nected with the positive terminal 

 of the dynamo or battery. When 

 the current passes, the silver in the 

 bath is thrown out of solution 

 and deposited on the teapot, while 

 at the same time an equal amount 

 of silver is dissolved off the anode 

 plate, and, entering into solution 

 in the bath, takes the place of that 

 deposited on the teapot. The pro- 

 cess goes on so long as the current is 

 maintained, until the anode is en- 

 tirely dissolved or until as much 

 silver has been deposited on the tea- 

 pot as is desired, when it is stopped. 

 The time occupied ranges from 

 two hours to twelve or even 

 longer, according to the work to 

 be done and the thickness of the 

 deposition or plating required. The 

 amount actually deposited on such 

 an article as a teapot is about 1 oz. 

 per square foot of surface covered, 

 the thickness of ordinary writing- 

 paper. The teapot, as it leaves the 

 bath, has a fine granular-looking 

 surface of chalky whiteness. The 

 smooth, bright finish of the shops 

 is given by polishing with wire 

 brushes kept 

 moist by stale 

 beer, after a 

 thorough wash- 

 ing in plain 

 water, dipping in 

 boiling water, 

 and drying in 

 not sawdust. 



While silver is 

 the chief metal 

 used in electro- 

 plating, others 

 commonly e m- 

 ployed are 

 copper, nickel, and gold. 

 Gold-plate is usually silver 

 electro-plated with gold. 

 During recent years the prac- 

 tice of plating iron with cop- 

 per has greatly developed; 



