Definition of Specific Impulse

Extract

We can agree with Professor Thorne that no physical quantity can be altered by varying the system of measurement, in the sense that the length of a ruler, for example, is the same whether its length be recorded in inches or centimetres. It is of course the numerical value of the length that varies with the system of measurement, and perhaps we should have used the words “numerical value” instead of “value” in the paragraph to which Professor Thorne refers.

We maintain our assertion, however, that in recording specific impulses, there is more likelihood of confusion as to numerical value, and as to the nature of the unit, if the mass definition is employed