NOTES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 165 



Arts and Sciences, which by liberal grants made from the Rumford Fund 

 of the Academy has given us substantial assistance in meeting the con- 

 siderable expense involved in this investigation. 



II. Description of the Apparatus. 



1. TJie Conductivity Cell or Bomb. — A vertical section of this is shown 

 in full size in Fig. 1. It is a cylindrical vessel, A, with a cover, B, 

 which is held in place by the large nut, C. A, B, and C are made of soft 

 crucible steel. To prevent contamination, the bomb is lined throughout 

 with sheet platinum 0.41 mm. thick. The cover joint is made tight by 

 a little packing ring, made of pure gold wire, which fits into a shallow 

 V-shaped groove. As may be seen in the diagram, the platinum lining, 

 indicated by a heavy line, goes under this ring and a little distance 

 beyond it, the outer edge being fastened to the shell by eight small steel 

 screws, of which two are shown. The lower vessel has a capacity of 

 about 122 ccm. 



The body of the bomb serves as one electrode, connection being made 

 with it by means of the large binding post on top of the nut C. The 

 second electrode is brought in through the bottom of the bomb and is 

 insulated from the latter by means of the mica washer, M, the air space, 

 S, and the quartz-crystal piece, Q. The body of this electrode is of 

 steel, but its upper part is covered with sheet platinum. On the bottom 

 of the crystal piece is turned a single sharp V-shaped ridge, and this 

 rests on a flat gold washer which is inserted between the crystal and 

 the bottom of the bomb. Another gold washer is placed between the 

 upper part of the electrode and a second V-shaped ridge turned on the 

 upper face of the crystal. The nut N, fitting on the lower, threaded 

 end of the electrode, draws the latter down, thus forcing: the ridges of the 

 crystal into the soft gold and making the joints tight. Z is a brass 

 washer which by its greater expansion-coefficient makes up for the 

 difference in the expansion, upon heating, of the quartz-crystal and of 

 that part of the steel electrode which lies within. The second nut, on 

 the lower end of the electrode, serves to bolt on a small copper tag to 

 which the wire Lg is silver-soldered. The quartz piece, Q, is extended 

 in the form of a cup above the electrode, so as to increase the resistance- 

 capacity of the cell. 



In the cover, B, is a narrow cylindrical chamber provided with an 

 " auxiliary electrode," which is insulated in just the same way as the 

 lower electrode. The purpose of this small chamber with the auxiliary 

 electrode is twofold : first, it serves as a safety device, showing that the 



