ATMOSPHERIC PRESSURE BETWEEN 104 C. AND 115 C. 403 



In order to maintain the same rise of temperature for different values of the steam 

 flow, it was necessary to alter both the number of accumulators and the rheostat. 

 With the flows used and for a rise of temperature of about 8 C., 9, 6 and 4 accumu- 

 lators were required for the maximum, medium and minimum flows respectively ; the 

 final adjustment of the current, to give a constant value of d9, being made by means 

 of the rheostat. This rheostat, which is also shown in fig. 5, consisted of pieces of 

 manganin wire (No. 20) each about 35 cm. long. By means of the arrangement of 

 mercury cups and the trough, indicated in the diagram, any number of these wires 

 could be connected in series or in parallel until the requisite current was obtained. 



The Standardised Resistance. 



This consisted of a coil of No. 22 manganin wire wound on a wooden frame which 

 was placed in oil contained in a double- walled vessel. A current of cold water was 

 passed through the space between the double walls, and the temperature was taken 

 by a mercury thermometer dipping into the oil ; this temperature was assumed to be 

 that of the coil itself. This secondary standard was compared with a 10-ohm primary 

 standard. The same current was passed through both, and the potential differences 

 across the ends of the resistances were obtained by the potentiometer. The results of 

 several comparisons, full details of which have been kept, show that the resistance of 

 the secondary standard is 5 '01 36 ohms at 17 C., and the temperature coefficient of 

 resistance 0'000020. 



The, Cadmium Cells. 



The cadmium cells used were of the H -pattern and were some of a set of twelve cells 

 which I made in 1906, the instructions given by JAEGER and WAOHSMUTH being 

 accurately followed. The mercurous sulphate and cadmium sulphate were supplied 

 by KAHLBAUM. Only seven of these cells have been used, Nos. 1 to 7 inclusive, in 

 those experiments in which seven cells were required, and Nos. 2 to 6, or 3 to 6 for 

 those experiments in which either five or four cells were required as a standard of 

 electro-motive force. During the experiments the cells were immersed in an oil-bath, 

 the current of water used for cooling the standardised resistance being first led round 

 the nest of cells through a compo-tube immersed in the oil. Cell No. 7 was sent to 

 the National Physical Laboratory to be tested in 1908, and again in 1913, and the 

 two reports give the values 1 '01875 volts at 17 C. in 1908 and 1 '01860 volts at 17 C. 

 in 1913. The N.P.L. comparisons were made with Weston normal cells, the E.M.F.'s 

 of which have been directly determined in terms of the international ohm and the 

 international ampere (10" 1 C.G.S.), and found to be 1'0183 international volts at 20 C. 

 During the 1913 standardisation the cell was put through a temperature cycle of 

 10 C., from 15 C. to 25 C., and though examined daily over a period of three weeks, 

 no anomalous changes of E.M.F. with temperature were recorded. All the cells used 



