CAPACITY FOR HEAT <>F MKTALS AT DIFFERENT TEMPERATURES. 125 



surrounding brass vessels. The reasons for discarding their use are given on 

 p. 159. 



A cylindrical hole (K), !) nun. in diameter and 14 cm. in length, was bored down 

 the centre of each metal block, co-axial with the central quartz tube. 



Into this was fitted t h " heating coil," the wire of which was wound on a light mica 

 frame of the X section used for platinum thermometers. The edges of the frame were 

 deeply serrated to prevent any possible contact between the wire and the surrounding 

 metal \v;ills. This hole was filled, at low temperatures, with liquid paraffin (previously 

 boiled and placed in vacuo to drive off volatile constituents), and at the higher 

 temperatures with a heavy hydro-carbon oil. 



Small mica " baffle plates " were inserted at intervals into the triangular sections 

 of the mica rack, in such a manner as to deflect the convection currents outwards. 



The hole O (depth 10 '3 cm., diameter I'l cm.), co-axial with the left-hand quartz 

 tube, contained one of the differential thermometers, the other being inserted into the 

 corresponding hole in the left-hand block. 



The position of the hole O was such that about half the total mass of metal was 

 contained in the annular ring whose outer surface was in contact with the copper 

 case, and whose inner passed through the centre of the hole. The various precautions 

 taken to secure accuracy in the use of these differential thermometers will be described 

 in Section III. 



The third hole (G) was used for the purposes of cooling the block below the 

 surrounding temperature, by the insertion of a thin-walled glass tube containing ether 

 and connected with a water pump. 



When the bath temperature was high the cooling process was a rapid one, but 

 somewhat tedious at lower temperatures. 



To prevent the entrance of laboratory air within the brass cases during cooling 

 which, by the deposition of moisture, might have had a serious effect, especially 

 when the tank temperature was C. a current of well-dried air was passed by 

 a branch tube into a larger one (F) which formed a continuation of the quartz 

 tube leading to the cooling hole. This rapid up-flow of dry air was continued until 

 the cooling tube had been withdrawn and replaced by a glass stopper, the lower end 

 of which reached within 3 cm. of the inner copper lid, and thus prevented convection 

 currents. 



Our methods of experiment involved measurements of the following quantities : 



(1) Temperature; 



(2) Resistance of heating coil ; 



(3) Potential difference at ends of heating coil ; 



(4) Mass ; 



(5) Time; 



(6) Thermal capacities of such bodies as oil, quartz, &c., whose temperature 



changed with that of the metal blocks. 



