Design Characteristics of Electromagnets for Telephone Relays

The electromagnets described are confined to relays, although the principles involved apply as well to selector magnets, clutch magnets and electromagnets in general. A treatment from the viewpoint of the telephone engineer is given of the i m p o r t a n t considerations which determine the design of the magnetic parts of relays and the economics of the winding dimensions. A knowledge of these factors as well as of the general considerations which are discussed is of great importance in the selection and application of relays to the telephone system. The operating and economic importance to the Bell System of the great number of relays required in the operation of the plant has been described in a previous paper.' INTRODUCTION L E C T R O M A G N E T S or relays as generally used in telephone switchboards are simply switches which are controlled electromagnetically. These switches may be required to open or close a number of separate and distinct circuits simultaneously or in a certain sequence. In many cases it is essential that the relay switch be opened or closed very quickly as this time may have a direct influence on the amount of apparatus required and consequently the first cost of the plant. The operating time of the relays also has a direct influence on the time required to establish a telephone connection. The above statements are particularly evident in automatic systems where selector apparatus is required to establish a connection between parties but is released during the conversation.