Meter original definition

Within the ionic medium convention, Sorensen s original definition (equation 29) may be used operationally because one can usually measure — log [H ] = —log H" rather accurately. An electrode system is calibrated with solutions of known concentrations of strong acid (e.g., HCIO4), which are adjusted with an electrolyte to the appropriate ionic strength, the observed potentiometer (pH meter) reading being compared with [H ]. 

By definition, photometers do not respond to radiation in the infrared or the ultraviolet (Fig. 4-4a). They are light meters in the sense that they mimic human vision that is, they respond to photons in the visible region, similar to the light meter on a camera. A candle is a unit of luminous intensity, originally based on a standard candle or lamp. The current international unit is called a candela (sometimes still referred to as a candle ), which was previously defined as the total light intensity of 1.67 mm2 of a blackbody radiator (one that radiates maximally) at the melting temperature of pure platinum (2042 K). In 1979 the candela was redefined as the luminous intensity of a monochromatic source with a frequency of 5.40 x 1014 cycles s-1 (A, of 555 nm) emitting 0.01840 Js-1 or 0.01840 W (1.464 mW steradian-1, where W is the abbreviation for watt and steradian... 

The unit of length or distance in the metric system is the meter. The meter was originally defined as 1 ten-millionth of the distance from the north pole to the equator through Paris, France. That is a rather difficult measurement to make, so later the meter was defined as the distance between two scratches in a special bar kept in a vault in Sevres, France. There is an even later definition, but we will be satisfied that it is the distance between those two... 

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some... 

The meter was originally defined, in the eighteenth century, as one ten-millionth of the distance from the equator to the North Pole and then, in the late nineteenth century, as the distance between two parallel marks on a special metal bar stored in a vault in Paris. More recently, for accuracy and convenience, a definition expressed in terms of light waves has been adopted. 

The definition of the meter has changed several times. Use an internet search engine (such as http //www. google.com) to locate the origin and various earlier definitions of the meter. Comment on some advantages of the present definition compared to earlier ones. 

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