Strain along the Lines of Force. 



687 



condensers is to avoid air-holes and fissures, as the electrodes 

 are only three millimetres apart. I did not accomplish this 

 until I used the arrangement shown in fig. 3, where the 



Fijr. 3. 



>Q filff fl/MP Afi/0 DftlEffS 



whole process takes place in a single bath and with a good 

 vacuum. The constituents, ground and thoroughly mixed, 

 were placed in the two litre-flask, A, in a bath o£ machine-oil 

 kept at a temperature a little above 110°. From the bottom 

 o£ the flask a glass and copper tube connects with the bottom 

 of the mould, B, through a stop-cock at C. A second tube 

 from the flask but above the surface of the mixture connected 

 the flask to a water vacuum-pump through driers ; and a 

 copper tube from the other end of the mould made connexion 

 with a " Geryk " vacuum-pump. The cock, C, was first 

 closed, and the mixture liquefied under a vacuum made by 

 the water-pump. This heating was continued until the mass 

 formed a homogeneous, smooth fluid. The " Geryk "" pump 

 was then started, and when a high vacuum in B was obtained 

 the cock, C, was opened and the fluid gradually forced into 

 the mould by regulating the pressure above the fluid in the 

 flask, — the " Geryk " was worked vigorously until the mould 

 was entirely filled. The pumps were then disconnected and 

 the bath allowed to cool slowly. 



Returning now to the completed casting, a stout frame 

 was made of two brass plates, connected by two brass rods 

 (fig. 2). The bottom electrode was fastened, as stated before, 

 to the base plate of this frame and it, in turn, to heavy beams 

 bolted to masonry piers in a basement room. A hole was 

 bored in the top plate of the frame and a brass cylinder 

 screwed into it so that the base of the cylinder was flush with 

 the lower surface of the plate and the upper end on a level 

 with the top of the ebonite tooth. The legs of a tilting 

 mirror were then placed on these upper ends of the cylinder 



