Negative electrification, 5. Non-conductors, 4. North pole, magnetic of earth, 40; of magnets, 26. Northern lights, 23. Ohm, the, 69. Open circuit cells, 50. Openers, for doors, 175. Outfits, dental, 175. Overhead trolley system, 166. Oxygen, attraction for hydrogen, 85. Parallel arrangement of lamps, 144. Peltier effect, 89. Pendant, electric, 151. Pith-ball electroscope, 5. Plate electrical machine, 10. Plates of cells, 45_a_. Plunge batteries, 53; large, 54. Polarity of coils, 95. Polarization, 84; electromotive force of, 85; of cells, 48. Pole-changing switch, 62. Poles, of cells, 45_a_; of horseshoe magnet, 26. Positive electrification, 6. Potential, defined, 65. Push-buttons, Chap. V.; indicating, 61; modifications of, 61; table clamp, 61. Quantity of electricity, 76; unit of, 76. Rays, cathode, 157; x-rays, 158. Receiver, telephone, 121. Reflectors, for lamps, 151. Registers, ink writing, 114. Relay, the, 113. Residual magnetism, 34. Resistance, coils and boxes, 69; electrical, 68; external, 68; and heat, 145; internal, 68; laws of, 70; unit of, 69. Retentivity, 34. Risers, in buildings, 139. Rods, lightning, 21. Roentgen, Prof., 158. Rosette, fusible, 142. Running-gear, of automobiles, 169. Safety, devices, 142; fuse, 142; fuse link, 142; fuse plug, 142; fuse ribbon, 142; fuse wire, 142. Search-lights, 153; signals sent by, 153. Secondary batteries, 86; uses of, 87. Series arrangement of lamps, 144. Series wound dynamo, 131. Service wires, 139. Shunt-wound dynamo, 131. Signal bells, magneto, 117. Simple cell, the, 45, 49. Single-fluid cells, 49. Single-point switch, 62. Single-stroke bell, 116. Socket, for incandescent lamps, 151. Soldering irons, electric, 147. Sounders, telegraph, 110; home-made, 110. Spark, effect of air pressure on, 155. Sparks, from cells, 17; from frictional electricity, 4. St. Elmo's fire, 22. Starting boxes, for motors, 163. Static electric machines, 8. Static electricity, condensation of, 15; electromotive force of, 17; to test presence of, 5; uses of, 14. Steam engines, in central stations, 170. Steel, inductive influence of earth upon, 43; retentivity of, 26. Storage batteries, the, and how they work, Chap. IX.; for automobiles, 169; for boats, 168; for natural sources of power, 87. Stoves, electric, 147. Strength of current, 71; measurement of, 73; unit of, 72. Switchboards, 62.