A step-up transformer with a circuit-breaker attachment. =Coils, Idle.= Coils in a dynamo in which no electro-motive force is being generated or developed. Coils that, through broken connections or short circuits, are inactive. =Column, Electric.= An old name for the voltaic pile. The apparatus made up of a pile of disks of copper and zinc, separated by pieces of flannel wet with acidulated water. =Comb.= A bar from which a number of teeth project like the teeth of a comb. It is used as a collector of electricity from the plate of a frictional electric machine. =Commutator.= An apparatus used on motors and dynamos and induction-coils for changing the direction of currents. It is made in a variety of types, but usually in the shape of insulated bars closely packed about an armature shaft. =Commutator-bars.= The metallic segments of a dynamo or motor-commutator. =Commutators, Quiet.= Commutators that do not spark during the revolutions of the armature. =Compass.= An apparatus for indicating the directive force of the earth upon the magnetic needle. It consists of a case covered with glass, in which a magnetized needle, normally pointing to the north, is balanced on a point at the centre. Under the needle a card is arranged on which the degrees or points of the compass are inscribed. A valuable instrument in electrical work, magnetism, etc. =Compass, Liquid.= A form of marine compass. The needle is attached to a card or disk which floats in alcohol or other spirits, so as to check undue oscillation. =Compass, Mariners’.= A compass in which the needle is attached to a card that rotates in pointing to the north. A mark, called the “lubber’s mark,” is made upon the case, and this is in line with the ship’s keel, so that a glance at the card will indicate the direction in which the ship is headed. =Compass, Spirit.= A form of mariners’ compass in which the bowl, or case, is sealed and filled with alcohol. The compass-card works as a spindle, and, by a series of air compartments, floats on the alcohol. The friction of the pivot is thereby greatly diminished, making the compass a very sensitive one. =Compass, Standard.= A compass employed as a standard by which to compare other compasses. =Condenser.= An appliance for storing up electro-static charges; it is also called a static accumulator. The telegraphic condenser consists of a box packed full of sheets of tin-foil having a sheet of paraffined paper or sheet-mica between every two sheets. The alternate sheets of tin-foil are connected together, and each set has its binding-post. (_See also_ Electrostatic Accumulator.) =Condenser, Air.= (_See_ Air-condenser.) =Condenser, Ayrton’s.= (_See_ Ayrton’s Condenser.) =Condenser-plate.= (_See_ Plate, Condenser.) =Condenser, Sliding.= An apparatus in the form of a Leyden-jar whose coatings can be slid past each other to diminish or increase the face area, and also to diminish or increase the capacity of the condenser.