Electrotyping 1 29 



IV. ELECTROTYPING. 



In the copying of models and small parts of apparatus it is 

 frequently of service to be able to copy an object in metal 

 successfully where casting would be impossible. The electrical 

 deposition of copper is, moreover, the only way in which satis- 

 factory electrical connection can be made between carbon and 

 metal surfaces, or quartz fibres and other parts of instruments 

 successfully mounted. 



In order to study the method, however, it will be well to 

 take some simple example, e.g. to make an electrotype copy of 

 a coin or medal. The method may then be applied to the 

 solution of any specific problem when it occurs. 



To make the mould, 1 soften a quantity of shredded 

 gutta-percha in hot water, and knead it into a pad free from 

 air-bubbles; place this in the middle of the surface of the 

 medal to be copied, and gradually press it towards the edges 

 in order to exclude all air between medal and mould. When 

 the edges are reached the gutta-percha is pressed tightly down 

 upon the medal, covered with a sheet of paper, a second sheet 

 being placed under the medal, the whole placed between two 

 wooden blocks, and tightly gripped in a vice till quite cold. 

 The mould is then hard and solid, but may be removed from 

 the object by bending slightly, when a perfectly bright and 

 smooth-surfaced mould should be obtained. A plaster cast 

 may be taken from this mould in order to test its completeness 

 and accuracy, as it is often difficult to detect slight differences 

 and deviations when the relief is reversed. 



For electrotyping, the surface of this mould is now to be made 

 electrically conducting. This is done by powdering a quantity 

 of ordinary blacklead, mixing to a paste with water, and smear- 

 ing the paste over the inner surface of the mould with a stumpy 

 camel-hair brush an ordinary one with two-thirds of the hair cut 

 away at right angles to the axis will serve the purpose admirably. 



1 A mould may he made in fusible metal by pouring it over the surface 

 of the warmed object. The copper is deposited all over such a mould, 

 and this is wasteful, but can be stopped by waxing the part not 

 required to be coppered. 



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