Practical. Thermometric Standard. 535 



The cumulative weight of evidence in favour of the para- 

 bolic difference-formula for the platinum resistance-ther- 

 mometer is very great, and proves that the adoption of this 

 formula as the basis of the practical standard scale would 

 give a A'ery close approximation to the theoretical scale. It is 

 important at the same time to give to the practical scale so 

 deduced a distinctive name, to prevent confusion with other 

 modes of expression. The time-honoured name of the British 

 Association, which has been in the past so closely connected 

 with other units and standards, appears to be at once the 

 most appropriate, and the most likely to command respect and 

 recognition. Temperature measurements referred to this 

 scale could be simply and clearly distinguished by affixing the 

 familiar letters B.A. ; and the results so expressed could be 

 easily corrected at any subsequent time when the relation 

 between the B.A. scale and the theoretical scale had become 

 more accurately known. 



(IV.) A peculiar advantage of the platinum resistance-ther- 

 mometer is the convenience of its practical application to all 

 kinds of experimental work. This arises chiefly from its 

 adaptability. A length of wire is cut off and disposed in any 

 manner which happens to be suitable for the particular object. 

 It is especially adapted for giving the mean temperature 

 throughout a space, and is, for this reason, most readily com- 

 parable with the gas-thermometer in spite of the large space 

 which the bulb of the latter instrument necessarily occupies. 

 Some instances of this adaptability have been brought before 

 the notice of the Section in various papers communicated at 

 the present meeting. In Mr. Bedford's experiments on the 

 expansion of porcelain, the mean temperature of the length 

 of the rod under observation was determined by means of a 

 platinum wire along the axis. Prof. Threlfall, in his gravity 

 balance, was similarly compelled to use a long platinum- 

 thermometer for observing the mean temperature of the 

 quartz thread, as he could not obtain mercury-thermometers 

 sufficiently sensitive and accurate for the purpose. As an 

 illustration of the extreme sensitiveness attainable, the high- 

 speed temperature-cycle diagrams from the cylinder of a gas- 

 engine, exhibited by Prof. Burstall, were an almost incredible 

 revelation of delicacy and accuracy. In the exact measure- 

 ment of small differences of temperature such as one thousandth 

 of a degree to within a few millionths, the observations of 

 Mr. Griffiths, described in connexion with the report of the 

 Electrolysis Committee, were remarkably verified by the 

 conductivity measurements of Mr. Whetham. A less sensa- 

 tional but equally important illustration was afforded by the 



2 P2 



