INDICATOR. Many kinds of apparatus are included under this name, which, in general, implies a device to reveal conditions not otherwise apparent to the eye or hearing. Thus depths in a tank are indicated by a float, either showing a rod above, or moving a chain over a pulley and sliding a pointer down the out side. The depth of a cage in a mine is revealed at the winding engine by a huge dial marked with figures, or in electric winders on a vertical scale with pointers travelling up and down. A scale is used to indicate the radius of a crane jib. Big dials on rolling mills show the distance apart of the rolls in inches and fractions.
Some machines have indicators to signify when a certain quan tity of work has been turned out. In others a bell rings when an automatic mechanism has used up a supply of raw material. A danger signal may be given by a bell in case of excess tempera ture. The copious forced oil-supply for the massive bearings and pinions of a rolling-mill has an alarm bell that rings should the circulation fail. A gas leak indicator is a very sensitive instrument, which will instantly show the presence of gas by the moving of a hand on a dial. Direction of movement is shown on some indi cators where risk might arise through a misunderstanding on this point. A speed indicator is a small instrument with a point to press against the end of a shaft, or a roller against a rim, to show the speed ; or it is a permanently fixed apparatus to indicate a rota tional speed of an engine or machine, or the travelling speed of a vehicle. A test indicator is used in machine-tool practice. It magnifies the movement of a button-ended spindle to make rec ords in thousandths of an inch on a dial ; very fine deviations from truth are thus indicated in the parts of machines or work, or differences in thicknesses of components can be rapidly gauged in a sort of caliper, one anvil of which is at the end of the in dicator spindle.
The pressure in the cylinder of a steam or explosion engine, or in a pump, compressor, or hydraulic machine, is ascertained with an indicator. This has a rotating drum turned by the pull of a cord from the cross-head (or in some cases the crankshaft) of the engine. Paper is wrapped round the drum, and a pencil connected by levers to a piston in a small cylinder traces a dia gram on the paper as the drum turns. A spiral spring of known strength is placed above the piston and the cylinder is connected by a pipe to the engine cylinder. The varying pressures in the latter therefore affect the pencil accordingly. The diagram is measured to ascertain the mean effective pressure, and from this the I.H.P. (indicated horse-power) can be calculated. Refined developments have taken place in indicators for high-speed en gines, including electrical recording, and optical types, which latter use a beam of light, tracing a diagram on a photographic plate. INDICATORIDAE : see HONEY-GUIDE.