498 BELL SYSTEM TECHNICAL JOURNAL 



tion of the underlying structural material which is usually a metal, 

 and not infrequently, (3) for electrical insulation purposes. Other 

 minor reasons for finishes exist on special apparatus. Many parts are 

 fashioned from such metals as steel, brass, aluminum and zinc alloys. 

 After such practices as punching and die casting the surfaces of these 

 parts are left in an unsightly condition, and furthermore unless pro- 

 tected, they may soon begin to corrode. In certain cases electroplated 

 finishes may be employed to advantage, but organic finishes, because 

 of their low cost and ease of application, find wide use. A good organic 

 finish for telephone apparatus must not only have a lasting decorative 

 value but must also protect the parts against the great variety of 

 conditions to which the apparatus is exposed. 



For example, the common black finish which is applied to various 

 parts of subscriber station apparatus, such as the zinc alloy handset 

 mounting, coin collector boxes and metal bell boxes must be suffi- 

 ciently tough and adherent to withstand perspiration, impact and 

 severe abrasion. Rigorous tests have been applied to find the most 

 durable finishes for such parts. They must maintain their appearance 

 so as to harmonize with the smooth molded black phenol plastic parts. 

 Advantage has been taken of the recent improvements in synthetic 

 resin finishes and a modified alkyd resin vehicle has been employed 

 in the present black enamel. A thorough baking is given to the enamel 

 which results in a more durable finish for telephone apparatus than the 

 former black japan. 



There are a number of applications for synthetic finishes where 

 corrosion protection is important from the standpoint of the proper 

 functioning of the apparatus. The aluminum diaphragms in marine 

 and aviation loud speakers and in sound power instruments are pro- 

 tected by a baked finish containing a heat-hardening phenolic resin 

 vehicle into which is incorporated a chemically inhibitive pigment. 



The familiar olive-green finish applied to the metal lining of tele- 

 phone booths is also a synthetic finish. This coating is often subjected 

 to unusual service conditions which only a modern type of finish can 

 withstand. Advantage is also taken of the high initial reflectivity 

 and the retention of light reflection of certain alkyd resins and these are 

 used in the white booth head-lining enamel. Synthetic finishes are 

 generally specified for' the finishing of Bell System trucks, etc. 



Other applications of finishes include lacquers and wrinkled enamels. 

 A recent interesting development has been the use of ethyl cellulose 

 dipping lacquers to form a continuous, fairly thick, envelope around 

 small telephone parts such as resistances, condensers and the like. 



