Apparatus and Test Fixtures for Automatic In-Circuit Electronic Testing of Electrical Components; Switching Equipment-Namely, an Electronic Exchange for Teletypewriter Networks Designed to Route, Connect, Disconnect, Time, Bill and Otherwise Control Calls Between and Among Teletypewriter Users Included in the Network; Electrical Bridges, Connecting Units for Telephone Signalling Equipment, Sealing Current Generators, Sealing Current Generators with Repeat Coils, and Relays; and Protective Circuits for Telephone Networks, and Mountings for Electrical Attenuator Circuits