NOTES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 179 



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. 

 lonof and 2 mm. wide, are inserted in the s{)ace left between the electrode 

 rod and the crystal, so that the ends of these mica pieces project per- 

 haps 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 hi 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. 



If, after the bomb has been in use for some time, it is necessary to re- 

 move the lower electrode, it may be done in the following way : The 

 nut, N, oannot be unscrewed, but enough of it can be removed with saw 

 and file so that the remainder will slip through the hole in the brass 

 washer. A light direct blow with the hammer on the end of the elec- 

 trode rod is then always sufficient to start it out. The brass remaining 

 in the threads of the electrode rod is easily removed with any pointed 

 tool, and the electrode is ready to use again. 



The cover, B, is made slightly concave to allow the air bubbles, which 

 mifht otherwise collect under it, to escape into the electrode chamber 

 above. To line the cover a round disk of platinum-iridium alloy con- 

 taininor 15 per cent iridium is taken, and the two tubes, T^ and Tj, are 

 soldered to this with pure gold. This alloy is used rather than pure 

 platinum on account of its greater hardness, which prevents the gold 

 ring from cutting into it. 



In the development of the bomb, the tube To has probably caused 

 more trouble than any other part. This is due in part to the fact that at 

 hitrh temperatures the pressure is sufficient to force water through the 

 linino' at any unsupported spot. If the tube was made of heavy metal, 

 and especially when it was made of the 2 per cent alloy, it was itself 

 capable, owing to its smaM diameter, of withstanding the pressure with- 

 out expanding enough to come into perfect contact with the steel at all 

 points ; as a result, the bomb would leak at such points. Or, owing to 

 the greater difficulty in mechanically expanding ^he small tube to meet 

 the shell, the fit would be so poor at the start that the hydraulic pressure 

 would tear it. Our earlier work here was done with the 2 per cent 



