A. F. BUBNS, in an article on testing of the crossbar system of telephony (Bell Lab. Rec., 20, No. 9 ; May, 1942), points out that in dialling systems a talking connexion is established through a large number (contacts, and it is desirable to test for ensuring tha. after the circuit is closed through to the called subscriber, there will be no open circuits in the offices through which the call passes. With the crossbar system, the sender makes a continuity test at the calling office and at the originating office, but it was found to be impracticable to provide a suitable routine testing procedure for the terminating office because of the multiplicity of paths that any particular call might follow. It was decided, therefore, to arrange the terminating marker to apply a continuity test for each call to make sure, before it is disconnected, that a circuit is completed to a subscriber line.