124 PROCEEDINGS OF THE AMERICAN ACADEMY. 



been specially purified was poured on top of the thimble at and 

 this was covered by a thin layer of a molten cement P which consisted 

 of a mixture of wood tar and shellac.^^ 



The tube is made to slip into the rotator arm and rests at the out- 

 side end on a disk of rubber packing. It is surrounded by oil to equal- 

 ize the hydrostatic pressure. A short piece of rubber tubing is slipped 

 onto the glass tube and turned back on itself as shown in the figure at 

 Q. The tube projects from the end of the arm far enough so that the 

 turned-back rubber just meets the end of the arm at D, and a piece of 

 "bill-tie" tubing is slipped over the rubber tubing and the arm to 

 prevent leakage of oil. 



The leads from the electrodes come out from the rotator in the 

 grooves E and are covered with small rubber tubing. Ordinary optical 

 tubing was not a good enough insulator, but insulation was used which 

 was stripped from 22-gauge rubber-covered wire supplied by the Simplex 

 Electrical Co. of Cambridgeport. The wires were attached to binding 

 posts on the rotator hub, from which electrical connection w^as carried 

 up to the mercury contacts. 



The Electrical Connections. 



The path taken by the electrical connections is indicated by dot and 

 dash lines in Figure 1. Leads from the binding posts on the rotator 

 hub pass up through the center of the hollow rotator and turbine shafts 

 and make connection with the mercury contacts. The wire used was 

 22-gauge rubber covered, as ordinary insulation would not stand the 

 severe conditions of temperature and the effect of moisture. 



At L in Figure 1 is a brass disk about 2-1/2 inches in diameter fas- 

 tened to the rotating shaft. This disk prevents any leakage of water 

 from the turbine trickling down the Hooke joint and getting into the 

 hollow rotator shaft and thus interfering with the insulation. is a 

 stationary pan about 10 inches in diameter with a central hole for the 

 shaft to pass through. This pan catches accidental dripping from the 

 turbine and protects the hub of the rotator and its binding posts. 

 Further protection is provided by a copper disk N with turned-down 

 edges. This disk rotates with the shaft and covers the central hole in 

 the pan 0. The disk also radiated the heat from the turbine and 

 from the friction of the steadying cords, which otherwise traveled down 

 the shaft and produced bad temperature differences between the cen- 

 tral and peripheral electrodes. 



^^ This useful cement was discovered by Dr. C. A. Kraus and Mr. R. D. 

 M alley of this laboratory. 



