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XV. An Instrument for the Comparison of Thermometers. 

 By "W.Watson, B.Sc, A.R.C.Sc, Assistant Professor of 

 Physics at the Royal College of Science, London *. 



IN most investigations where it is necessary to measure 

 temperature it is convenient to use a small thermometer, 

 often of short range, particularly suited for the purpose. 

 The only practicable method of determining the errors of such 

 a thermometer is by comparing its readings with those of a 

 standard thermometer. This comparison is in general ren- 

 dered very difficult on account of the two thermometers 

 differing both as to the length of the scale and the capacity 

 of the bulb. Hence if, as is essential, the bulbs are placed 

 close together in a water-bath, a considerable length of one 

 of the mercury columns must be out of the bath, involving an 

 uncertain correction for cool column. Again, the difference 

 in size of the bulbs renders the lag of the thermometers 

 different, so that unless the temperature of the bath is kept 

 absolutely constant for some time we cannot be sure that 

 both thermometers have reached a steady temperature. 

 These objections make the use of a water-bath at tempera- 

 tures over about 20° C. both troublesome and unsatisfactory, 

 and this paper is written in order to describe an instrument 

 which the author has designed for comparing some thermo- 

 meters required for some other work, and which has worked 

 so satisfactorily that it may be of some use to others. 



The principle employed is that of using a vapour-jacket 

 in order to obtain different constant temperatures, as recom- 

 mended by Ramsay and Young (Journ. Chem. Soc. xlvii. 

 p. 640, 1885). A general view of the instrument is shown 

 in fig. 1. A glass tube AB, closed at the top and open 

 below, about 76 cm. long and 2 - 5 cm. in diameter, passes up 

 inside a second glass tube of about the same length and 

 4*7 cm. in diameter. The space between these two tubes is 

 at the bottom closed by an indiarubber cork C (figs. 1 and 2 ), 

 while at the top the inuer tube is kept in place by three 

 small pieces of glass rod fused on and forming a triradiate 

 star. The upper end of the outside tube is fused on to a 

 Liebig condenser D, the upper end of the condenser being 

 connected by a rubber joint fitted with a glycerine seal to a 

 manometer E, a stoppered funnel F, and a large glass bottle 

 which acts as a reservoir. This bottle is packed round 

 with cotton-wool and is enclosed in a wooden box G. The 



* Communicated by the Physical Society : read May 14, 1897. 



