APPENDIX 1. Units and numbers

When electrophysiologists measure currents, voltages cr concentrations they measure physical quantities that are the same physical quantities that a physicist or chemist might deal with. In order to characterize precisely these entities it is necessary to specify two things:

1. (1) The quality of the units, and

2. (2) The numerical size of the units used.

Dimensions

We shall deal first with the quality of physical units and this quality is called the dimension. Natural scientists have agreed on a number of different systems of units. The two most widely used systems are the c.g.s. system in which the fundamental units are:

1. (L) length – centimetre

2. (M) mass – gram

3. (T) time – second

and then there is the MKS system with the fundamental units of:

1. (L) length – metre

2. (M) mass – kilogram

3. (T) time – second

The latter is the so-called rationalized MKS system, more closely related to the International System of Units (SI), which is coming steadily into universal use.

In Table 1 we give some physical units that are used in the two systems. More detailed information is available in Quantities, Units, and Symbols, a report by the Symbols Committee of the Royal Society, 2nd edn, 1975, and in Units of Measurement, Preprint from the Geigy Scientific Tables, 7th edn, 1968 (Basle, Switzerland: Geigy).