=Door-opener, Electric.= A magnetic contrivance arranged in connection with a lock, by means of which the latch is released by pressing a distant push-button. This device is used in flats and apartment-houses for opening a door from any of the apartments in the house. =Double Filament Lamp.= An incandescent lamp having two filaments, one with a high capacity, the other with a low one. The high capacity may be from sixteen to fifty candle-power, the other from one to five. A turn of the bulb in its socket, or the pulling of a string which operates a switch in the socket, cuts out the current from the long filament and sends it through the shorter and finer one, thus giving a weaker light. These “hy-lo” lamps are useful as night lamps in halls, bath-rooms, or in sick-rooms, where a low or weak light is required all night. =Double Pole-switch=. A cut-out that is arranged to cut out the circuit of both the negative and positive leads at the same time. =Double-push.= A contact-push having two contacts and arranged so that pressure upon it opens one contact and closes the other. =Double Throw-switch.= A switch so arranged that it can be thrown into either one of two contacts; a throw-over switch. =Driving-pulley.= The broad-faced or channelled pulley on an armature shaft by means of which the power from a motor may be transmitted mechanically. =Dry Battery.= (_See_ Battery, Dry.) =Duct.= The space in an underground conduit for a single wire or cable. =Duplex Wire.= An insulated conductor having two distinct wires twisted or laid together, but properly insulated from each other. =Dynamic Electricity.= Electricity in motion or flowing, as distinguished from static or frictional electricity. Electricity of relatively low potential or electro-motive force in large quantity or amperage. =Dynamo.= An apparatus consisting of a core and field-magnets, properly wound with insulated wire, which, when put into operation by revolving the core or armature at high speed, develops electric current; a mechanical generator of electricity. =Dynamo, Motor.= (_See_ Motor-dynamo.) E =Earth.= The accidental grounding of a circuit is termed an “earth.” =Earth-plate.= A plate buried in the ground to receive the ends of telegraph lines and other circuits, and so give a ground connection. Copper plates are often used, but in houses the ground is usually formed by attaching a wire to the gas or water pipes. =Earth Return.= The grounding of a wire in a circuit at both ends gives the circuit an earth return. This method is commonly used in telegraph lines, both in the wire and wireless systems. =Eddy Currents.= (_See_ Currents, Eddy.) =Edison Distributing-box.= A box used in the Edison “three-wire” system, from which the outlets pass to local circuits. =Edison Lalande Cell.= A zinc-copper battery having a depolarizing coating of copper oxide on the copper element, the couple being immersed in an electrolyte composed of potash or caustic soda.