201,687. English Electric Co., Ltd., and Bolton, R. A. R. May 24, 1922. Automatic circuit-breakers; protective arrangements.-In direct-current circuit breakers of the kind in which the movable contact member comprises a current carrying part carrying a magnetic armature end which is held closed by an electromagnet and is moved to interrupt or partially interrupt the main current by the electromagnetic and/or electrodynamic actions of the main current upon the magnetic and/or current-carrying parts of the movable contact-member, a switch of small inertia is provided to facilitate the opening by breaking or inserting resistance in the holding circuit, this switch being operated in accordance with the value or rate of charge of the main current. As applied to a circuit breaker of type described in Specification 176,547 (Fig. 1), the pivoted movable contact member 4 carries an armature 3 arranged in the gapsbetween adjacent ends of two U-shaped cores 1, 2 the former carrying a main current winding 1 and the latter a separately-excited holding coil 13. The winding 2 is in series with the fixed and movable switch contacts 5, 7 and with a blow-out coil 8, and a resistance 17 bridges themain contacts so as to be inserted in the main circuit when they separate. The holding coil 13 is in series with a resistance 19 an auxiliary switch 16 which is opened by a projection 15 on the circuit-breaker contact arm 4 after the main contacts separate maintaining open the holding circuit until the push button 18 is depressed, when the coil 13 is fully energized to close the circuit-breaker. Pivotally mounted on two lugs 24 projecting from the core 1, Figs. 2 and 3, is a switch of small inertia consisting of a frame 20 - - 23, the two side members 20 and 21 being of magnetic material which carries a switch arm 25 bearing a bridging contact 26 co-operating with contacts 27, 28 in the holding circuit. Normally the attraction of the bar 21 to the core 12 assisted by the spring 29 maintains contacts 26 - - 28 in engagement, but on excessive current flowing in the coil 2, the attraction of the bar 20 to the core 1 overcomes these forces, so breaking the maintaining circuit and permitting a rapid opening of the main contacts 5, 7. This opening action is assisted by the electromagnetic action of the main current on the armature 3 and the magnetic circuit of the folding coil and by the electrodynamic action of the current in the movable contact member 4 and the adjacent fixed main current-carrying conductors. In a modified form, Fig. 4, in which the switch of small inertia is separate and responds in accordance with the rate of change of current, its energizing coil 32 operating contacts 31 which control contacts 30 in the folding circuit is fed from the secondary winding 33 of a transformer 34 whose primary winding 35 is in series with the coil 2. Instead of using a separate transformer a secondary winding 36 may be wound on the core 1 to energize the coil 32. To prevent the choking action produced by such a winding on the flux in the core 1 this winding may be wound on two inward projections of the core 1. e.g. 38, Fig. 5, which then carry the winding 37 supplying the switch winding 32. A demagnetizing winding 39 on the core 12 may be connected in series or shunt to the windings 32, 37 to supplement the action of the small inertia switch. As a further alternative. the winding 32 may be connected in series with the winding 2, a resistance 40 being shunted across the whole or a part of this winding and so arranged that the time constant of the branch through the winding 40 is considerably less than that through the other main current branch, so ensuring that the operation of the switch contacts 30 depends to a considerable extent on the rate of change, of the main current.