Measuring Instruments

In physics and engineering, measurement is the activity of comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the measurement results in a given number for the relationship between the item under study and the referenced unit of measurement. Measuring instruments are the means by which this translation is made. Physicists use a vast range of instruments to perform their measurements. These range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators.

Balance

A balance (also beam balance or laboratory balance) is used to accurately measure the mass of an object. This class of measuring instrument uses a comparison technique in its conventional form of a beam from which a weighing pan and scale pan are suspended. To weigh an object, it is placed on the measuring pan, and standard weights are added to the scale pan until the beam is in equilibrium. Very precise measurements are achieved by ensuring that the fulcrum of the beam is friction-free (a knife edge is the traditional solution), by attaching a pointer to the beam which amplifies any deviation from a balance position; and finally by using the lever principle, which allows fractional weights to be applied by movement of a small weight along the measuring arm of the beam. While the word "weigh" or "weight" is often used, any balance scale actually measures mass, which is not dependent upon the force of gravity, as opposed to a scale with a spring, which measures weight. Mass is properly measured in grams, kilograms, pounds, ounces, or slugs; while weight is in newtons or pound force.

Length

An altimeter is an active instrument used to measure the altitude of an object above a fixed level. For example, a laser altimeter can measure height from a spacecraft above an ice-sheet. That measurement, coupled with radial orbit knowledge, will enable determination of the topography. The traditional altimeter found in most aircraft works in measuring the air pressure from a static port in the airplane. Air pressure decreases with altitude - about one millibar per 27 feet close to sea level. The altimeter is calibrated to show the pressure directly as altitude in accordance with a mathematical model defined by the International Standard Atmosphere (ISA). The reference pressure can be adjusted by a setting knob. This is nescessary since sea level air pressure varies with the weather. In pilot's jargon, the regional or local air pressure at mean sea level is called the QNH, and the pressure which will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field. An altimeter cannot however be adjusted for variations in air temperature. Difference in temperature from the ISA model will therefore cause error in indicated altitude. In a spreadsheet, the calibration formula for an altimeter (up to 36090 feet) can be written as: h =(1-(P0/Pref)^0.19026)*288.15/0.00198122 where h is the indicated altitude in feet, P0 is the static pressure and Pref is the reference pressure (use same units for both). Other types of altimeter are the radar altimeter that measures the altitude more exactly using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure the exact height during the landing procedure of commercial aircraft.

Area

A planimeter is a drafting instrument used to measure the surface area of an arbitrary two dimentional shape. The precise way in which they are constructed varies, the main types of mechanical planimeter being Polar; Linear; and Prytz or "Hatchet" Planimeters. They rely on Green's theorem and were invented by the Swiss mathematician Jacob Amsler in 1854. Electronic versions also exist.

Angles

A sextant is a measuring instrument used to measure the angle of elevation of a celestial object above the horizon. Making this measurement is known as sighting the object or taking a sight. The angle, and the time when it was measured, are used to calculate a position line on a nautical or aeronatical chart. A common use of the sextant is to sight the sun at noon to find one's latitude. See celestial navigation for more discussion. The scale of a sextant has a length of 1/6 of a full circle; 60°, hence the sextant's name. An octant is a similar device with a shorter scale, 1/8 of a circle; 45°, which was in use until 1767 when it was quickly replaced by the sextant. In 1767 the first edition of the nautical almanac tabulated lunar distances, enabling navigators to find the current time from the angle between the sun and the moon. This angle is however sometimes larger than 90°, and thus not possible to measure with an octant. Sir Isaac Newton invented the principle of the doubly reflecting navigation instrument, but never published it. Two men independently rediscovered the sextant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), an American inventor. The sextant replaced the astrolabe as the main instrument for navigation.

temperature

The traditional method of measuring human body temperatures was with a Mercury-in-glass thermometer whose tip was inserted either into the mouth (oral temperature), under the armpit (axilliary temperature) or into the rectum via the anus (rectal temperature). Oral temperatures may only be used on patients that are capable of holding the thermometer correctly in their mouth securely, which excludes small children or people who cough too much or vomit (this is now less of a problem with fast-reacting digital thermometers, but was certainly an issue with mercury thermometers taking minutes to register a temperature). Another counter-indication is if the patient has drunk a hot or cold liquid beforehand, in which case one has to wait or use another method. In the case of rectal temperatures, insertion, especially by another person than the patient, should be preferably done with lubrication such as petroleum jelly (now discouraged) or a water-based personal lubricant. Rectal temperatures are the most precise method, in general, but in some cultures (such as the United States) they are found too embarrassing to be applied to patients older than a few years. Rectal temperatures are still often considered the method of choice for infants. In the 1990s, mercury thermometers were found too risky to handle and have largely been replaced with electronic thermometers, or, more rarely, thermometers based on liquids other than mercury. Both kinds may be used orally, axilliarily, or rectally. Other kinds of medical thermometers exist: tympanic thermometers measure the temperature of the tympan by infrared measurement; band thermometers are applied to the patient's brow.