Practical Physics is a free textbook on basic laboratory physics. See the editorial for more information....  # Arbitrary Units at Present Employed

For many of the quantities referred to in the table (p. 18) no arbitrary unit has ever been used. Velocity, for instance, has always been measured by the space passed over in a unit of time. And for many of them the physical law given in the second column is practically the definition of the quantity; for instance, in the case of resistance, Ohm's law is the only definition that can be given of resistance as a measurable quantity.

For the measurement of some of these quantities, however, arbitrary units have been used, especially for quantities which have long been measured in an ordinary way as volumes, forces, &c.

Arbitrary units are still in use for the measurement of temperature and quantities of heat; also for light intensity, and some other magnitudes.

We have collected in the following table some of the arbitrary units employed, and given the results of experimental determinations of their equivalents in the absolute units for the measurement of the same quantity when such exist:

Table of arbitrary units
Quantity Arbitrary unit employed Equivalent in absolute units
Angle Degree (1/180 part of two right angles)
Quantity of beat Amount of heat required to raise the temperature of unit mass of water one degree The gramme-centigrade unit is equivalent to 4.214 107 ergs