NOTES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 177 



the subsequent operations that the work is not crowded too hard 

 against the abrading surface, as this causes a local rise of tempera- 

 ture which may crack the crystal. A hollow drill, whose internal 

 diameter is but little greater than the external diameter of the finished cup, 

 is then run through the crystal piece at right angles to the sawed surfaces. 

 Such a drill consists merely of a tinned-iron tube provided with a slit 

 running lengthwise, and mounted so that it can be rotated in the lathe. 

 The outer end of the tube is turned off square and is then charged with 

 diamond powder. A small piece of wet sponge is then placed in 

 the tube. Powdered carborundum can be used in place of diamond, and, 

 although it is somewhat slower in starting, it appears to be equally satis- 

 factory afterwards. The core is then taken from the drill and the ends 

 of the cylinder are ground down flat ; for the saw has left them somewhat 

 irregular. This operation of grinding is conveniently carried out by 

 means of a carborundum wheel rotated in the lathe, the wheel being kept 

 wet by holding a sponge against it. Before grinding either surface, its 

 bounding edges must be ground off (bevelled) ; otherwise the edges will 

 break out irregularly. To cup out the cyliuder, a hollow drill, whose 

 external diameter is but little less than the internal diameter of the 

 finished cup, is then run into one end to a depth almost equal to that of 

 the desired cavity. The core which is left from this drill is too strong 

 to be broken out without clanger of injuring the outside of the cup; so 

 another, smaller one is next run in to the same depth as the first and con- 

 centrically with it. This leaves two fragile pieces, a small rod and a thin 

 tube, which are easily broken out. The cup is next mounted so that 

 it can be rotated in the lathe. This is best accomplished by fastening it 

 with stick shellac to the end of a brass rod held in the lathe chuck. To 

 hold firmly, the crystal must be heated above the melting-point of the 

 shellac; this can be done safely by flashing it with a gas flame. The 

 inside of the cup is then ground to its final diameter and the bottom made 

 flat by using carborundum powder upon the end of a brass rod which is 

 a little less in diameter than the cavity and whose end is squared off, the 

 rod being best held in the hand. The 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 halfway through from either end. It is afterwards expanded to its 

 proper size by the aid of a small brass or steel rod and some carborun- 

 vol. xxxix. — 12 



