When Professor Thomson began his work as a teacher in the University of Glasgow, there was, as has already been noticed, great vagueness of specification of physical quantities. Few of the formal definitions of units of measurement, now to be found in the pages of every elementary text book, had been framed, and there was much confusion of quantities essentially distinct, a confusion which is now, to some extent at least, guarded against by the adoption of a definite unit, with a distinctive name for each magnitude to be measured. Thus rate of working, or activity, was confused with work done; the condition for maximum activity in the circuit of a battery or dynamo was often quoted as the condition of greatest efficiency, that is of greatest economy of energy, although it was exactly that in which half the available energy was wasted. Partly as a consequence of this vagueness of specification, there was a great want of knowledge of the values of physical constants; for without exact definitions of quantities to be determined, such definitions as would indicate units for their measurement, related to ordinary dynamical units according to a consistent scheme, it was impossible to devise satisfactory experimental methods to do for electricity and magnetism what had been done by Regnault and others for heat. The first steps towards the construction of a complete system of units for the quantitative measurement of magnetic and electric quantities were taken by Gauss, in his celebrated paper entitled _Intensitas vis magneticæ terrestris ad mensuram absolutam revocata_, published in 1832. In this he showed how magnetic forces could be expressed in absolute units, and thus be connected with the absolute dynamical units which Gauss, in the same paper, based on chosen fundamental units of length, mass, and time. Thus the modern system of absolute units of dynamical quantities, and its extension to magnetism, are due to the practical insight of a great mathematician, not to the experimentalists or "practicians" of the time.