NOTES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 



181 



end of T x on an anvil, the soft gold, projecting perhaps 3 mm. ahove the' 

 steel at V, is compressed with a riveting hammer. The asbestos is now 

 drawn out of the tube, and a hole is drilled down through the gold to 

 meet the hole in the platinum. 



The valve seat at the start is of the form shown in Fig. 1, and the 

 slightest pressure on the steel bicycle-ball serves to force the ball down 

 into the soft gold tube enough to make the joint tight. But after this 

 process has been repeated a few times the bearing surface of the ball 

 on the gold becomes so large that the pressure which can be obtained 

 by tightening the screw, K, with the fingers is not sufficient to make the 

 joint tight. The valve seat can be easily brought back to its original 

 condition, however, by filling the depression, which the ball has made, 

 with soft dental gold and opening the hole again with a scratch awl 

 or a drill. The filling is most conveniently accomplished with a little 

 "moss fibre" gold, using the regular dental tool. Of course the top 

 should always be left concave, so that the ball will of itself roll to the 

 center. Time is saved in the end by putting the valve seat in order 

 each time; for, if this is done, there will never be a leak at this point. 

 It is also better to use a fresh steel ball each time. The screw, K, 

 should fit well, but still turn easily with the fingers ; for, if there were 

 much friction here, it would be impossible to tell how much of the force 

 employed in screwing it down was being communicated to the ball and 

 how much was wasted iu friction in the screw. Smearing a little vase- 

 line on the screw prevents air from leaking iu while the bomb is being 

 evacuated. 



The edge of the lining is fastened to the cover by eight steel screws, 

 in the same way as the flange of the lining of the body of the bomb is 

 secured, the only difference being 

 that it is not necessary at the top 

 to recess the edge of the platinum. 

 Care must, of course, be taken 

 that the screws in the top do not 

 come opposite those in the lower 

 part when the top is put on. 

 Two reference marks enable the 

 experimenter to bring the cover 

 always into the same position with 

 respect to the bottom. 



The cover lining is forced into place and tested, just as was the-lining 

 of the lower part of the bomb, by means of the Cailletet pump, making 



Figure 6. 



