276 PROCEEDINGS OF THE AMERICAN ACADEMY. 



indicating roughly the amount of pressure exerted by the water within 

 the apparatus, some parts of which were not well calculated to bear a 

 great disruptive force. 



The copper wire w, about 0.016 cm. in diameter, leads from a point 

 on the upper copper coating of the disk through a hole in h, where it 

 is held in a water-tight joint between a hard rubber peg and a soft 

 rubber sheath surrounding the peg, to a wooden shelf, where its end is 

 gripped between two copper washers under the head of a screw, all of 

 which objects are shown in the figure. A thicker copper wire, about 

 0.06 cm., one end of which is pinched between the upper washer 

 and the head of the screw, leads off toward a galvanometer. Another 

 thin copper wire, w', similar to w, starting from a corresponding point 

 on the under copper coating of the disk, passes through // and is 

 fastened at its outer end exactly as the wire w is fastened. It is 

 necessary to consider these details, for one can easily imagine a 

 method of connecting the fine wires with the thick wires that would 

 give rise to very disturbing thermo-electric forces and make the whole 

 investigation useless. 



At first the copper wires w and w' were soldered to the copper faces 

 of the disk, as little solder as possible being used, with the hope that 

 the thermo-electric forces at the top and bottom of the very thin layer 

 of solder used would be very small in comparison with the large 

 thermo-electric forces due to the copper-steel junctions. But tins hope 

 was not justified. The disturbance arising from these soldered junc- 

 tions was very serious. The fact appears to be that each copper wire, 

 exposing on the whole a surface of many square millimeters to the 

 water, kept its point of attachment with the solder much warmer or 

 much colder, as the case might be, than the same point 

 would have been kept by direct contact with the water. 

 The effect is similar to that observed when a soldered 

 — junction between two copper wires, one thick and one 

 thin, is held a short distance beneath the surface of 

 Fig. 2. water differing considerably in temperature from the 

 air above it. In such a case, represented by Figure 2, 

 the submerged part of the thin wire, and also its point of attachment 

 with the layer of solder, has very nearly the same temperature as the 

 water ; but the submerged part of the thick wire does not so nearly 

 attain this temperature. The result is a difference of temperature 

 between the two points of attachment with the solder, and a conse- 

 quent thermo-electric effect, which an unwary observer might attrib- 

 ute to difference in the thermo-electric quality of the two wires. If 



