[Illustration: Fig. 662.--Interior view of Columbia watt hour meter (type D), showing construction and principal parts and connections. The armature winding consists of three coils approximately circular in shape. The coils are form wound, interlocked with one another and with the light impregnated fibre disc which serves as a spacer for them. The aluminum damper disc has the conventional anti-creep provision in the shape of the three small soft iron plugs, mounted close to the central staff, which the illustration shows. These in their revolution come successively within the influence of an adjustable iron screw which is magnetized by an extension from one of the damper magnets. The angular relationship of the armature windings and of the three iron plugs is such that at the time that the armature is exerting a maximum torque the magnetized screw is exerting the maximum pull to hold back a given plug and conversely when the armature pull is a minimum the magnetic screw is attracting a plug with the maximum effort to cause ahead rotation. The irregularities of torque are in this way smoothed out. The commutator has three segments and is made of chemically pure silver. Each brush is formed of a length of phosphor bronze wire bent like a hair pin and secured at its "U" end to a brass sleeve, which in turn is secured to an insulated stud by a set screw. An extension on the sleeve carries a micrometer screw brush adjustment. The main speed adjustment is secured by providing a soft iron bridge plate, bridging over the extremities of each magnet end and adjustable, by means of a set screw and lock nut, to any desired distance therefrom. This gives a regular micrometer means of varying the effective magnet strength. Interposed between the series coil and the permanent magnets is a heavy soft iron shield to guard the magnets against disturbance by short circuiting. Light load adjustment is obtained by providing in the coil circuit a series of small resistance spools, equipped with pin terminals, to which connection can be selectively made by means of a split bushing terminal on a flexible cord. This series of spools is strung on a metal arbor located within the case.] [Illustration: Fig. 663.--Diagram showing internal connections of the Duncan watt hour meter. Its operation depends upon the principle of the well known electro-dynamometer, in which the electromagnetic action between the currents in the field coils and an armature produces motion in the latter. It also embodies the other two necessary watt hour meter elements required for the speed control and registration of the revolutions of the armature, these being embodied in the drag magnet and disc, and the meter register respectively. The motion of the armature is converted into continuous rotation by the aid of a commutator and brushes, the commutator being connected to the armature coils and carried on the same spindle therewith.]