178 PROCEEDINGS OF THE AMERICAN ACADEMY. 



dum. To turn the little V-shaped ridges on the ends of the cup, a 

 diamond is set in the end of a steel rod just as for the boring tool, except 

 that here a larger rod may be used to advantage. The tool is held in 

 the hand and supported on the T-rest just as the ordinary hand tool is 

 used on metal. The outside of the cup is first turned true with the 

 diamond, and the ridges are ground out by holding a flat piece of car- 

 borundum (the face of a carborundum wheel answers nicely) against it as 

 it revolves. 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 ensures getting the ridges, as they should 

 be, in parallel planes and centrally located with reference 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. 



In this connection it may be mentioned that some ready formed cups, 

 made of fused silica, have lately been furnished us by Siebert & Kuhn 

 of Cassel, Germany. All that remains to be done to these cups is to 

 drill the small hole in the bottom and turn the ends. They have not 

 yet been tried, but unless they are more acted upon by hot water than 

 those of quartz-crystal, which seems unlikely, they will be preferable for 

 several reasons. For, in the first place, the making of such a cup from 

 crystal is a somewhat tedious operation, and, in the second place, a very 

 rapid cooling of the bomb has to be avoided, as the crystal would other- 

 wise crack. Even with the fused-silica cups it will doubtless be neces- 

 sary to cool the bomb slowly to avoid loosening the joint, but the cup 

 would not be lost even if the bomb happened to be cooled suddenly. 

 Another advantage of the fused silica is that, owing to its small expansion 

 coefficient, there would be a much smaller change in the cell constant 

 with the temperature (see Section VIII). 



The tliickness of the brass compensating washer (Z, Fig. 1) can be 

 calculated from the known coefficients of expansion of the quartz-crystal 

 and of tlie brass and steel used. That used in our bomb was 5.1 mm. in 

 thickness. 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 



