the Titer mat Unit. 451 



Electrical Standards committee appointed by the Board of 

 Trade (July 1891), when the length was increased to 



106'3 centim. 



I have recalled these familiar facts in order to show how 

 great has been the advantage of proceeding on a sound 

 theoretical basis. We have here a change in the practical 

 standard amounting to 1J per cent., but the disturbance 

 caused by the successive corrections has been small, for the 

 fundamental basis of the whole system has remained unchanged. 

 Had the unit of resistance been arbitrarily selected and de- 

 pendent on the behaviour of some particular conductor, which 

 further was subject to changes with whose nature we were 

 but dimly acquainted, each successive alteration would have 

 damaged the whole superstructure, and the progress of elec- 

 trical science would have been hampered and retarded. Why 

 not proceed in an analogous manner with regard to a system 

 of thermal measurements ? viz., first define an ideal unit 

 based on a system of absolute measurements and of a con- 

 venient magnitude for practical use, bearing in mind condi- 

 tions (A) and (C) supra. 



Let us next apply the information we already possess for 

 the purpose of making a first approximation to the physical 

 measurement of the unit thus selected, and, to use the words 

 of Professor Callendar, " leave it to a more enlightened and 

 skilful generation " * to make a second, and if necessary (as 

 in the case of the ohm) a third approximation. 



It would, I think, be advisable to give the ideal thermo- 

 dynamic unit some distinctive title. Unfortunately the most 

 appropriate one (a Joule) has already been annexed!, and 

 such being the case the ideal unit might with great propriety 

 be termed a "Rowland" J, at all events I propose to make a 

 temporary use of the name in that sense. 



I employ the name " therm ,3 to indicate " the quantity 

 of heat required to raise 1 grin, of water (measured in vacuo) 

 through 1° C. of the nitrogen-thermometer of the Bureau 

 International at a temperature of t° C. by that thermometer.'" 

 Thus the value of a therm would be a function of the tem- 

 perature. 



The " ROWLAND " should be of such a magnitude that a 

 therm at some convenient temperature (say within the range 

 10° to 20° C.) would be its heat equivalent. The particular 



* Phil. Trans. 1887, A, p. 195. 

 f A Joule = 10 7 ergs. 



% The names " meyeb" and "kelvin" also naturally suggest them- 

 selves as both appropriate and convenient. 



