286 PROCEEDINGS OF THE AMERICAN ACADEMY. 



nearly uniform thickness, though one of them was left slightly thicker at 

 the edge. The final thickness of each coating was not far from 0,2 cm. 

 The convex surface of the disk was turned off sufficiently to leave a 

 good surface and show clearly the junctions of the copper coatings with 

 the iron body. The diameter of the disk was thus reduced to 9.94 cm. 



Each coating was now channelled at the edge to a depth of about 

 0.1 cm. and a width of about 0.17 cm,, as iu Figure 1 of Plate I., and 

 a brass ring, Ji or H', 0.3 cm. thick, was shrunk into the channel iu 

 each coating. R' is cut through in Figure 1. The object of this detail 

 will appear presently. 



Mounting and Use of the Disk. 



Figure 2 shows how the disk was placed and used in the experiments 

 on conductivity. In tliis figure, the scale of which is -^-, I represents the 

 disk; c and c' are the copper coatings; and the rings just described can 

 be seen set into the edges of the coatings. The lower ring is shown cut 

 through by horizontal passages. There are, in fact, in this ring 24 

 horizontal slots, each about 1 cm. long and 0.2 cm. wide, the object of 

 which is to allow the water entering vertically beneath the middle of the 

 disk to flow out horizontally from beneath the disk, thus carrying away 

 the air-bubbles which warm water inevitably contains, and which would 

 accumulate beneath the disk if an immediate downward escape of the 

 water through small passages were required. Upon passing from be- 

 neath the disk the water enters a groove cut in a hard rubber ring, h'h', 

 and covered by a brass flanged ring n n fastened to h'h'. Thence it 

 passes downward and out of the apparatus by several passages of 4 or 

 5 millimeters iu diameter, only two of which are shown in the figure. 

 The slotted brass ring through which the water flows carries the iron 

 disk, and rests in a groove in the hard rubber ring h'h', a soft rubber 

 tube at the bottom of this groove making a water-tight packing. The 

 ring h'h' has at the bottom another groove, which receives the top of the 

 brass ring r'r', which rests upon a wooden support to which it is firmly 

 attached by means of a horizontal flange. Soldered within r'r' near its 

 top is the brass plate p'p', which carries the hard rubber block H' H', in 

 the centre of which is fixed the tube that carries the water to the bottom 

 of the disk. This tube is enclosed for a part of its length by another, 

 which extends downward from p'p' ; but this is an unimportant detail. 



Encircling the iron disk is a soft rubber band b h, which was intended 

 partly as a protection of the iron against rusting, and partly as a dam to 

 prevent leakage of water upward past n?i. Another similar band, not 



