CAPACITY FOR HEAT OF METALS AT DIFFERENT TEMPERATURES. 131 



from the tank, as, if nearer, it responded to the Hashing on and off of the heating 

 lamps, the changes in the magnetic system of the chronograph, &c. It stood on the 

 top of a massive pillar of masonry which passed through the laboratory floor without 

 contact, and whose foundations were embedded deeply below the base of an under- 

 ground rhamlx-r. Tlu- traffic in ( 'anliil' is heavy, but by taking special precautions, 

 the galvanometer in these circumstances was but little affected. We found it 

 necessary, however, carefully to guard the system against convection currents. 

 Every small opening near the suspension was blocked with slips of mica, and the 

 whole galvanometer was enclosed within two separate chambers. Many of the 

 oscillations usually attributed to earth vibrations are, we believe, due to insufficient 

 attention to the effect of convection currents. 



As all our temperature measurements were observed in terms of lengths of the 

 bridge wire, it is evident that the accuracy obtainable was dependent upon the accuracy 

 of the calibration of that wire. We have notes of a calibration made some 14 years 

 ago. Before these observations were begun a careful re-calibration was made in terms 

 of the "mean unit" of the marble-top box (A) previously referred to. The d'Arsonal 

 galvanometer used on that occasion, however, was not sufficiently sensitive to enable 

 the determination of the smaller inequalities. 



The calibration was made in terms of 3 coils in Box A, of the approximate value of 

 1, 5, and 10 hundredths of an ohm. Near the conclusion of our present work, a very 

 careful re-calibration was conducted, with the object of ascertaining the accuracy of 

 the earlier one and also of ascertaining if the bridge wire had suffered any alteration 

 through use. The Paschen galvanometer was employed ; two separate and inde- 

 pendent calibrations were conducted by the two observers and the results were in 

 remarkable agreement. 



It appeared that the calibration over the longer intervals on the former occasion 

 was correct, thus showing that the wire had not suffered in the interval. 



Each unit of the wire was then expressed in terms of a " mean Box A unit " (the 

 same \init as that used in the standardisation of the thermometers), and a table was 

 formed showing the value of a bridge wire unit at regular intervals, in terms of one 

 Pt degree of each pair of thermometers. 



It should here be stated that until the final steps in the reduction of our results, 

 all temperatures are expressed in the platinum scale. 



SECTION IV. 

 Resistance of Heating Coil. 



Our methods of reduction demanded a knowledge of R under the actual conditions 

 prevailing during an experiment. As it was impracticable to stir the oil in which the 

 coil was immersed, a wire of small temperature coefficient was chosen to reduce to its 

 smallest limits the correction for the heating effect of the current on the wire. 



8 2 



