HALL. — CONDUCTIVITY OF MILD STEEL. 275 



The parts dd and d'd' are disks of brass, the edges of which press 

 upon the rings hh and h'h'. These disks are perforated near the edge 

 by numerous holes for flow of water. The disks are packed water- 

 tight, or nearly so, in the brass rings b b and b'b', small rubber tubes 

 being used in the joints. 



The parts g g and g'g' are blocks of hard rubber secured by the nuts 

 n and n' to the brass tubes t and t'. The object of these blocks is to 

 guide the flow of the water across the surfaces of the disk in order to 

 make the temperature over each surface as nearly as possible every- 

 where the same. 



The block (j'g' is not fastened to the disk d'd' upon which it rests, 

 but is carried by the tube t' in such a way that when t' is rotated g'^ 

 turns with it. It was the practice at certain stages of the work to 

 rotate l' briskly back and forth through a number of degrees, in order 

 to facilitate the escape of air-bubbles from the space above and around 

 g'g'. Numerous small holes drilled through the upper and outer edge 

 of g'g' were intended to make, and probably did make, this operation 

 more effective, 



Most of the water (80 to 85%) coming up through t' passed down 

 and out by way of the double funnel f'f, but in order to remove the 

 air-bubbles a certain amount of overflow was maintained through seven 

 small tubes leading obliquely upward" and outward from the water 

 space at the edge of g'g'. One of these overflow tubes is shown in 

 the figure at the left. Each of them was connected with a small 

 rubber tube, and the outer end of each rubber tube was about 2 cm. 

 higher than the top of hh. Whenever the tube t' and the block g'g' 

 were being agitated to remove air-bubbles, as before described, the 

 main outlet o' was closed, so that all the water which passed under 

 the disk had to escape by the overflow tubes. This method of getting 

 rid of air-bubbles was adopted after the problem had been studied by 

 means of a kind of dummy apparatus, in which the disk »S'was replaced 

 by a plate of glass. There is reason to believe that it was fairly 

 effective, but it was cumbrous, and probably a different arrangement 

 will be used in future work. No similar device appeared to be 

 necessary for the space immediately above the disk, as air-bubbles 

 originally there would naturally be carried upward through the tube 

 t by the escaping water. A single overflow tube, q, led out from the 

 double funnel //, by which the water was brought to the upper 

 surface of the disk, and through this tube a small overflow was 

 maintained to carry off bubbles that might otherwise have caused 

 trouble. Another function of q was to serve as a kind of gauge 



