=Astatic Needle.= A combination of two magnetic needles so adjusted as to have as slight directive tendency as possible. The combination is generally made up of two needles arranged one above the other with the poles in opposite directions--commonly called “Nobili’s Pair.” These needles require but a slight electro-force to turn them one way or the other, and are used in astatic galvanometers. =Atmospheric Electricity.= (_See_ Electricity, Atmospheric.) =Atom.= The ultimate particle or division of an elementary substance. Electricity is largely responsible for the presence of atoms in the atmosphere. =Atomic Attraction.= The attraction of atoms for each other. Principally due to electric disturbance. =Attraction.= The tendency to approach and adhere or cohere which is shown in all forms of matter. It includes gravitation, cohesion, adhesion, chemical affinity, electro-magnetic and dynamic attraction. =Aurora.= A luminous electric display seen in the northern heavens. It is commonly thought to be the electric discharges of the earth into the atmosphere, due to revolution of the former and to the heat produced at the equator. As compared to the static machine for generating frictional electricity, the earth represents the revolving wheel gathering the current and discharging it at the poles. =Automatic Cut-out.= An electro-magnetic switch introduced into a circuit, so as to break the circuit of the latter should it become overloaded with current; it also acts in the event of a mechanical interruption. =Automatic Regulation.= A speed regulator worked by electricity so that a uniform flow of current may be secured automatically. =Ayrton’s Condenser.= This is a pile of glass plates separated by small pieces of glass at the four comers, so that the plates cannot touch each other. Tin-foil is pasted on both sides of every plate, and the two coatings are connected. The tin-foil on each second plate is smaller in area than that on the others, and the plates are connected in two sets, negative and positive. In this construction it will be seen that the glass is not the dielectric proper, but acts only as the plane to which the tin-foil is pasted. One set of plates are connected to a binding-post by strips of tin-foil, and the other set are connected to another binding-post in a similar manner. B =B.= An abbreviation for Beaumé, the inventor of the hydrometer scale. Thus, in speaking of the gravity of fluids, 20° B. means twenty degrees Beaumé. =Back Induction.= A demagnetizing force produced in a dynamo when a lead is given to the brushes. (_See also_ Induction, Back.) =Back Shock.= A lightning stroke received after the main discharge. It is caused by a charge induced in neighboring surfaces by the main discharge. =Bad Earth.= A poor ground connection, or one having comparatively strong electrical resistance. =Balance.= A proper adjustment between the apparatus and the electro-motive force, thus securing the best possible results.