NOTES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 173 



it bids fair to wear indefinitely, and yet it is not so brittle that it cannot 

 be forced into the groove in the steel without cracking. The flange 

 could probably just as well be welded to the platinum cup, but in our 

 bomb it was soldered to it with pure gold. Pure gold was also used 

 freely in making repairs on the present lining when it tore, as it fre- 

 quently did at the start before the necessity of the small holes in the 

 shell and of several other precautious was understood. In the measure- 

 ments made so far the gold has exerted no deleterious influence. 



The lining is made so as to fit as well as possible at the start. It is 

 then inserted in the shell, the latter is placed in the lathe, and by means 

 of a steel or agate burnisher the lining is spun in. This brings the 

 lining for the most part in contact with the steel, but nothing can be 

 done at the bottom in this way. By means of a piece of soft wood and 

 a mallet, however, the bottom also can be made to fit closely. The 

 flange at the top is now spun over in the lathe. The stiff alloy is easily 

 and safely brought down into the sharp groove by proceeding as follows : 

 One end of a short piece of brass rod 3 or 4 mm. in diameter is filed to 

 a sharp V. Using this and a small hammer, the platinum can be driven 

 down almost to the bottom of the groove. After every half dozen 

 strokes the brass V will have to be sharpened. The brass is consider- 

 ably softer than the alloy, and so does it no damage. Other methods 

 have been tried, such as forcing a ring of hard-drawn copper wire down 

 onto the platinum over the groove by means of the cover B and the 

 large nut C. But the former method, although a little tedious, is much 

 better. The sides of the groove in the steel make an angle of 90° with 

 each other, and its depth is such that when a wire 0.8 mm. in diameter 

 is laid in it, about one half of the wire lies outside the groove. 



After the lining has been made to fit as closely as possible, it should 

 be removed from the shell and heated to redness to anneal it. Even the 

 'flange had better be treated in this way, since it is hard enough even 

 after annealing. To remove the lining after it has been fitted in in 

 the preceding manner, the following plan was adopted : Take a stick of 

 soft wood, perhaps 20 cm. long and 5 cm. square, and whittle one end 

 down so that it will slip easily into the bomb. Then take a piece of 

 cotton cloth moistened with alcohol to remove any grease, wrap it over 

 the small end of the stick, and then with a hammer drive the latter 

 tightly into the bomb. Now holding the bomb in the vise, grasp the 

 projecting end of the stick firmly in the hands and twist out the lining. 

 This method never fails, provided the steel shell was ground reasonably 

 smooth at the start. 



