PLASTIC MATERIALS IN TELEPHONE USE 499 



This frequently eliminates a potting operation and provides an ex- 

 cellent mechanical protection for the parts. 



The employment of organic coatings for insulation purposes is 

 another important application deserving mention. In general the 

 conditions in the telephone plant are not such as to demand resistance 

 to very high voltages. Millions of feet of copper wire receive a baked 

 clear enamel coating applied in multiple thin coats to assure maximum 

 flexibility and uniformity. Carefully chosen pigmented alkyd baking 

 enamels are used in exchanges as insulating finishes for various hooks, 

 bars and other small parts of the metal framework upon which the ex- 

 change wiring is tightly and compactly fastened and from which elec- 

 trical insulation is needed. 



An appreciable amount of clear cellulose acetate lacquer is at present 

 used on switchboard wire. This is applied as a thin coating of a 

 specially plasticized lacquer over a layer of textile insulation, the latter 

 being colored in various ways for ready identification. The require- 

 ments of a good lacquer coating material for switchboard wire are 

 chiefly (1) low cost, (2) reasonably good insulation, even under pro- 

 longed high humidity conditions such as occur during the summer 

 months in many parts of the country and (3) good transparency so as 

 not to alter the identification colors on the textile serving. Smooth- 

 ness, flexibility, inflammability and corrosion hazard are other important 

 factors that receive consideration. 



A synthetic plastic which has recently found a small but important 

 place in the telephone plant is polybutene. When coated on fabric 

 this plastic has given an excellent membrane material for a new type 

 of handset transmitter. It is dust and moisture-proof, light in weight, 

 flexible and alkali resistant, not impairing in any way the acoustical 

 properties of the instrument. 



Synthetic Resins as Adhesives 



A growing use for synthetic resins in the telephone plant is in the 

 form of adhesives. The amount of material consumed in this way is 

 not large but the applications are frequently important from the stand- 

 point of the functioning of the apparatus as well as from the economies 

 involved. The older kinds of adhesives such as casein and animal 

 glue are still employed for joining together various large parts (es- 

 pecially wood, as in cabinet work, etc.) but they are brittle and gener- 

 ally unsatisfactory in the assembly of small light parts (metal, phenol 

 fiber, ceramic, etc.) such as go into special communication apparatus. 



