Practical Physics is a free textbook on basic laboratory physics. See the editorial for more information.... 
Home Physical Measurements Arbitrary and Absolute Units  
Search the VIAS Library  Index  
Arbitrary and Absolute Units
The method of measuring a quantity, q [Q], is thus resolved into two parts: (1) the selection of a suitable unit [Q], and (2) the determination of q, the number of times which this unit is contained in the quantity to be measured. The second part is a matter for experimental determination, and has been considered in the preceding chapter. We proceed to consider the first part more closely.
There are likewise two wellrecognised standards of mass, viz. (1) the British standard pound, a certain mass of platinum kept in the Standards Office; and (2) the kilogramme des Archives, a mass of platinum kept in the French Archives, originally selected as the mass of one thousandth part of a cubic metre of pure water at 4°C. One or other of these standards, or a simple fraction or multiple of one of them, is generally selected as a unit in which to measure masses by any observer making mass measurements. The kilogramme and the pound were carefully compared by the late Professor W. H. Miller; one pound is equivalent to 0.453593 kilogramme. With respect to the unit of time there is no such divergence, as the second is generally adopted as the unit of time for scientific measurement. The second is 1/86400 of the mean solar day, and is therefore easily reproducible as long as the mean solar day remains of its present length. These units of length, mass, and time are perfectly arbitrary. We might in the same way, in order to measure any other physical quantity whatever, select arbitrarily a unit quantity of the same kind, and make use of it just as we select the standard pound as a unit of mass and use it. Thus to measure a force we might select a unit of force, say the force of gravity upon a particular body at a particular place, and express forces in terms of it. This is the gravitation method of measuring forces which is often adopted in practice. It is not quite so arbitrary as it might have been, for the body generally selected as being the body upon which, at Lat. 45°, gravity exerts the unit force is either the standard pound or the standard gramme, whereas some other body quite unrelated to the mass standards might have been chosen. In this respect the gallon, as a unit of measurement of volume, is a better example of arbitrariness. It contains ten pounds of water at a certain temperature. We may mention here, as additional examples of arbitrary units, the degree as a unit of angular measurement, the thermometric degree as the unit of measurement of temperature, the calorie as a unit of quantity of heat, the standard atmosphere, or atmo, as a unit of measurement of fluid pressure, Snow Harris's unit jar for quantities of electricity, and the B.A. unit of electrical resistance.


Home Physical Measurements Arbitrary and Absolute Units 