Robots are used for pilot less air crafts, deep diving, dealing with nuclear fuel, rescue, fire fighting, surveillance, aerial photography, to detect chemical and biological agents in war, space probe, study of extra terrestrial life, at extreme weather conditions, doing work at 24/7 level etc.

Minimum distance to hear an echo is 17 m. The effect of sound on ear remains for 0.1 second. If the sound is reflected back in less than 0.1 second, no echo is heard.

Horse shoe magnets can retain magnetism for a long time if keeper is used with magnet while storing.

Psychrometer measures humidity and it consists of dry & wet bulb thermometers. Take the difference of reading of the two thermometers and refer the humidity chart to get humidity in percentage.