=Anti-hum.= A shackle inserted directly in a line-wire near a pole. It is provided with a washer or cushion of rubber to take up the vibrations of a wire. To continue the circuit a bridle, or curved piece of wire, is connected with the line-wires that are attached to the shackle. =Arc.= A term applied to an electric current flowing from carbon to carbon, or from metals separated by a short gap, as in the arc street-lamps. The original arc was produced by two vertical rods, through which the current passed up and down. When not in action the upper ends touched, but as the current flowed the ends were separated, so that the current, passing up one carbon across the gap and down the other, formed the segment of a circle in jumping from one tip to the other. An arc of electric flame is of brilliant and dazzling whiteness. The voltaic arc is the source of the most intense heat and light yet produced by man. The light is due principally to the incandescence of the ends of carbon-pencils, when a current of sufficient strength is passing through them and jumping over the gap. Undoubtedly the transferred carbon particles have much to do with its formation. The conductivity of the intervening air and the intense heating to which it is subjected, together with its coefficient of resistance, are other factors in the brilliant light produced. =Arc-lamp.= An electric lamp which derives its light from the voltaic arc, by means of carbon-pencils and a current jumping from one to the other. =Arc, Quiet.= An arc free from the hissing sound so common in arc-lights. =Arc, Simple.= A voltaic arc produced between only two electrodes. =Armature.= A body of iron or other material susceptible to magnetization, and which is placed on or near the poles of a magnet. That part of an electric mechanism which by magnetism is drawn to or repelled from a magnet. The core of a dynamo or motor which revolves within the field magnets, and which is the active principle in the generation of current by mechanical means, or in the distribution of power through electrical influence. Armatures are sometimes made of steel, and are permanent magnets. These are used in magneto-generators, telegraph instruments, and other apparatus. =Armature-bar.= An armature in a dynamo or motor whose winding is made up of conductors in the form of bars. =Armature-coil.= The insulated wire wound around the core of the armature of an electric current-generator or motor. =Armature-core.= The central mass of iron on which the insulated wire is wound; it is rotated in the field of an electric current-generator or motor. =Armored.= Protected by armor; as cables may be surrounded by a proper sheathing to guard them from injury. =Astatic.= Having no magnetic directive tendency, the latter being a general consequent of the earth’s magnetism. =Astatic Circuit.= (_See_ Circuit, Astatic.) =Astatic Couple.= (_See_ Couple, Astatic.)