Section 5. Construction of the Bomb. ip 



small hole is drilled through the bottom of the cup by the aid of a small 

 diamond set in the end of a steel or brass rod. The diamond must, of 

 course, be a little larger than the rod to give clearance for the latter. To 

 start the hole the T-rest is used, but afterwards the rod is supported only 

 by the hand. The tool must be withdrawn and moistened very frequently. 

 The hole may be run half-way through from either end. It is afterwards 

 expanded to its proper size by the aid of a small brass or steel rod and 

 some carborundum. To form the little V-shaped ridges on the ends of the 

 cup, each of the end surfaces, except at the middle where the ridge is to 

 be, is ground down with carborundum. The projecting portion left in the 

 center is then turned into a sharp ridge by means of a diamond set in the 

 end of a steel rod. This tool is held in the hand and supported on the 

 T-rest just as the ordinary hand tool is used on metal. To support the cup 

 while work is being done on the lower end it is best to fasten in the chuck 

 a piece of brass rod somewhat smaller than the internal diameter of the 

 cup, square off its end, turn a little groove in it which will correspond to 

 the ridge at the bottom of the cup, and then shellac the cup on, so that the 

 ridge comes in the groove. This mode of support insures getting the 

 ridges, as they should be, in parallel planes and centrally located with ref- 

 erence to the axis of the cup. The operation of polishing is best carried 

 out by means of different grades of corundum powder, using finally oxide 

 of tin. These are applied wet on the end of a soft piece of wood. 



In making such a cup an ordinary mechanician, after a little practice on 

 the different operations, will spend perhaps twelve or fifteen hours. 



The thickness of the brass compensating washer (Z, fig. 1) can be cal- 

 culated from the known coefficients of expansion of the quartz-crystal and 

 of the brass and steel used. That used in our bomb was 5.1 mm. in thick- 

 ness. Of the two gold washers the upper one is made to fit tightly on the 

 electrode rod, while the hole in the lower one is made to correspond with 

 that in the bottom of the bomb. To keep the lower gold washer from 

 touching the electrode rod, and to keep the latter from touching the steel 

 shell, the following device was employed : The middle part of the steel 

 rod is made about 0.5 mm. smaller in diameter than the hole in the crystal. 

 Three thin strips of mica, each about 2 cm. long and 2 mm. wide, are 

 inserted in the space left between the electrode rod and the crystal, so that 

 the ends of these mica pieces project perhaps 1 cm. below the cup. The 

 mica strips are cut so wide that they have to be pushed into place. They 

 serve to hold the rod in the crystal and keep the lower gold washer in 

 place. The cup can now be grasped by its edge with a pair of tweezers 

 and the electrode rod pushed through the hole in the bottom of the bomb. 

 It is then bolted down. It is next tested to make sure that there is no short 

 circuit between the electrode and the bomb ; and finally, to make sure that 

 the joint is tight, the bomb is connected once more to the pump. 



