316 Mr. H. H. Poole on the Rale of 



laid on the lid o£ the inner ice vessel and completely buried 

 in ice. For about two feet more the leads are encased in 

 rubber ; most of this portion is also in the ice. 



Between the couple and the galvanometer is placed a 

 reversing-key. It consists of two blocks or! wood each about 

 3 inches square by 2 inches deep. In the centre of the 

 lower face of one block and of the upper face of the other 

 were bored holes about one inch in diameter and one inch 

 deep into which molten lead was run. The ends of the 

 lead cylinders so formed were finished off flush with the 

 surfaces of the blocks. The bared ends of the leads from 

 the couple are stuck down to one lead cylinder, being 

 insulated from it by paper soaked in molten paraffin- wax. 

 The leads to the galvanometer are similarly stuck to the 

 other block in such a way that by laying one block on the 

 other the bared wires come into contact and close the circuit. 

 By turning the upper block through 90° the circuit is 

 opened, and a further turn through 90° reverses the original 

 connexions. The whole is placed under a bell- jar and the 

 upper block lifted and turned by means of a glass rod passing 

 through the stopper. In this key thermal effects are reduced 

 to a minimum, the lead blocks serving to equalize the tempe- 

 rature of the points of contact ; the resistance of the key was 

 found to be negligible. 



A suspended coil mirror-galvanometer is employed. It is 

 enclosed in a fireclay cylinder to minimize thermal effects at 

 its terminals. Even so such effects are never quite absent, 

 but are eliminated by the system of reading employed. The 

 circuit is always left open, and before making an observation 

 the open circuit reading of the galvanometer is taken. This 

 changes from clay to day, and seems to depend on the tempe- 

 rature of the cellar in which the apparatus is set up. The 

 circuit is then closed and a reading taken ; the circuit is 

 reversed and another reading taken. The difference in 

 reading produced by reversal is entered as the deflexion, and 

 from it the temperature of the calorimeter is obtained. 

 Thermal effects at the galvanometer only render the de- 

 flexions on each side of zero unequal and do not affect the 

 total deflexion produced by reversal. 



Calibration of Thermo-couples* 



The couples when used with this galvanometer are too 

 sensitive to be conveniently compared with a mercury 

 thermometer. Two methods of calibration were employed. 

 One method consisted in calibrating the galvanometer by 



