Section 5. Construction of the Bomb. 21 



drilling is extremely difficult. Owing to capillary forces, no gold will go 

 to the space inside until the smaller spaces between the convolutions are 

 all filled, so that there is no danger so long as too much gold is not em- 

 ployed. In this, as in all other operations when gold is used in soldering 

 platinum, the piece should be kept hot no longer than is absolutely neces- 

 sary, because the gold rapidly alloys with the platinum, and the resulting 

 alloy is more crystalline in structure than either of the constituents and has 

 not their ductility. To make the joint between 7\ and the lining of the 

 cover stronger, the tube is reinforced above this point, as shown by the 

 drawing. This was necessary in our earlier apparatus before the lining 

 was screwed down to the cover, but is probably not necessary in the later 

 form. 



As the upper end of the tube 7\ is to act as a valve-seat, and as there- 

 fore there will be a good deal of downward pressure at this point, the tube 

 has to be well expanded into the conical cavity in the steel at V (fig. 1). 

 Because of this, and of the further fact that the valve-seat should be as 

 soft as possible, it is better to make the upper end of 7\ of solid gold, bor- 

 ing it out later. This is done as follows : The tube is first packed full 

 of asbestos, to prevent gold from getting into it. A band of thin platinum 

 foil is next wound tightly around the upper end and bound on by means 

 of a platinum wire. This band is then pushed partly off of the end of the 

 tube, so as to make a small projecting tube; and pure gold is melted into 

 this until it is full. The platinum foil on the outside of the gold is now 

 filed off. Both tubes are now attached to the cover lining and inserted in 

 place in the cover. Holding the lower end of 7\ on an anvil, the soft gold, 

 projecting perhaps 3 mm. above the steel at V, is compressed with a rivet- 

 ing 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 slight- 

 est 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 fiber" 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 



