AND LIQUIDS AND THEIR VARIATION WITH TEMPERATURE. 



401 



two discs of copper, C, "103 centim., and U, '312 centim. thick, with a flat heating 

 coil of double silk-covered No. 25 platinoid wire, P, between them, well insulated by 

 means of mica and shellac, the total thickness of the coil and mica being '110 centim. 

 The contacts between the surfaces of the copper discs U and M, and the substances, 

 were improved by smearing them with glycerine and sliding them together. The 

 glycerine was generally sufficient to keep the discs together during an experi- 

 ment, but to ensure the surfaces remaining properly in contact, a few turns of silk 

 thread were wound round the discs. 



At opposite extremities of a diameter of each disc, two holes, '7 millim. diameter 

 and 3 millim. deep, were bored towards the centre of the disc, and into one hole of 



Fig. 4. 



Fig. 5. 



Fig. 7. 



each disc a platinoid, and into the other hole a copper, wire was soldered. The 

 wires were double silk covered, about 30 centims. long, and were all cut from the 

 same two coils. 



The surfaces of the pile of discs were varnished to give them the same emissivity, 

 and the pile placed in the centre of a double-walled air-bath (fig. 6), of 17 centiras. 

 height, 15 centims. length, and 7 centims. breadth, supported on legs about 4 centims. 

 above the table. The top of this bath consisted of a double wooden lid, cut down 

 the centre to allow the wires from the discs to pass out. Th edges of the lid were 

 covered with green baize, in which the wires embedded themselves, and which 

 closed the bath sufficiently to prevent convection currents between the inside 

 and outside. On the outer walls of the air-bath a coil of insulated platinoid wire 

 was wound, and an electric current could be sent through the wire to keep the 

 bath at any required temperature. The inner walls were about 1 centim. within 

 the outer, and between the two water could be circulated, or the space could be filled 

 with air simply. A thermo-j unction was soldered to the centre of an inner surface, 

 and enabled the temperature of the enclosure to be determined. The wires from the 

 heating coil, P, after passing out through the lid, dipped into mercury cups, from 

 which wires passed to the cells supplying the heating current, the power absorbed 

 being regulated by a rheostat and measured by a wattmeter. 



VOL. cxci. A, 3 F 



