Improvements in or relating to telephone systems

818,098. Automatic exchange systems. AUTOMATIC TELEPHONES & ELECTRIC CO. Dec. 2, 1957 [Dec. 14, 1956], No. 38158/56. Class 40 (4). In a graduated howler circuit, the discharge of a condenser C1 through a high resistance R2 varies the gain of a tube VB in order to pass the output of a tone oscillator VA with progressively increasing amplitude. The supply unit HSU, which may serve the whole exchange, is connected with a number of distribution circuits DC one on each first selector rack. When a cord connection is made between jack J and the access path of a selector which is held by an offending subscriber relay X, energizes over the sleeve followed by ST. Relay HT starts the oscillator VA and condenser C1 charges rapidly. After an interval relay Y pulls up and C1 discharges slowly, allowing tube VB to conduct after about 20 seconds and the gain to increase to a maximum after a further 12 seconds. When the subscriber hangs up, ST falls back and cuts off the tone and lamp LP 1 lights. If connection is made to a second jack J while the unit HSU is in use C1 recharges so that the tone output ceases and then builds up afresh.