Improvements in or relating to telephone systems

940,034. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd., and STANDARD TELEPHONES & CABLES Ltd. Oct. 30, 1961 [Nov. 17, 1960], No. 39498/60. Heading H4K. [Also in Division G4] Individual stores on a magnetic drum record individual subscriber's meter totals and when a metering pulse identifying a particular subscriber is detected his total is increased by one, the stores being arranged in groups of tracks and corresponding tracks in each group being scanned simultaneously with the other groups and sequentially through the group, each track being provided with a test meter store, the total in which is increased by one at each scan and is compared with the total in the corresponding test store in another group. Since a discrepancy detected in a comparison of test totals in two groups does not locate the fault in one group or the other, comparisons are made between each group and two others. Thus a fault involving groups 1 and 3 and groups 3 and 4 locates the fault in group 3. Each register track of the magnetic drum serves 100 lines or junctions and the tracks are arranged in groups of eight which are scanned sequentially to serve corresponding groups of 800 lines or junctions. Each group of eight register tracks has a carrier track and a meter check track in common. As shown in Fig. 3, the write head of a meter track is placed the length of two meter stores ahead of the read head and the read head of the carrier track is placed two meter stores ahead of the write head. If a metering instruction is received it is synchronized with the appropriate metering total store by means of a library track which identifies the line or junction involved. The total existing in the store is read out over the track switching circuit and meter track read circuits to be written by way of carrier track write circuit on to the carrier track. After a delay of 98 meter storage lengths the carrier track record of the total is passed from the carrier track read circuit to an add 1 circuit enabled by the metering instruction and from which the increased total is written back on to the meter track. in the same position as the original total. At the same time as the increased total is written on to the meter track the original total from the carrier track is written on to a meter check track or this total is increased by one in a second add 1 circuit. Two store lengths later both the meter track total and the meter check track total are read out to a meter path chec