I 



CROSSBAR DIAL TELEPHONE SWITCHING SYSTEM 



89 



SO that when the relay is operated satisfactory contact is made between 

 the metal surfaces of the magnetic circuit which insures uniform time 

 characteristics. 



In both the U and Y type relays the cyhndrical cores permit the use 

 of form wound coils which are wound on special machines and slipped 



Fig. 6 — "U" type relay. 



over the cores when completed. In the manufacture of these coils a 

 removable mandrel is used. It is covered with a layer of sheet cellu- 

 lose acetate and accommodates several coils. These coils are then 

 automatically wound on the mandrel from different spools of insulated 

 wire. Separations are left between adjacent coils so that when the 

 winding operation has been completed the individual coils can be sepa- 

 rated. A very thin sheet of cellulose acetate is automatically inter- 

 leaved between successive layers of wire to hold the wire in place and to 

 provide insulation between layers. This general method of winding 

 coils also is used for the magnets of the crossbar switches and multi- 

 contact relays. 



Functions of the Equipment Units 



The general operation of the system as a whole may be more easily 

 understood by first describing the principal equipment units in the 

 system and their functions before proceeding with a description of the 



