272. Index mechanism of water-meter, 37. Indicator of electric bell, 119. Induction coil, 121; uses of, 125. Injector, 39; Giffard's, _41_; principle of, 40; self-starting, 42. Interlocking of signals, 204, 222. Internal-combustion engine, 87. Iris of eye, but permitting the gas is a third to.