BISHOP. — THERMAL CONDUCTIVITY OF LKAD. 677 



Means of Different Sets of Readings. 



Thermo-Electromotive Force. 



The thermo-electromotive force per degree was determined by plac- 

 ing the hemispheres in two adjacent tanks, connecting them with a 

 piece of the original lead, and maintaining these tanks at a constant dif- 

 ference of temperature, this difference of temperature being measured by 

 means of two Beckman thermometers, one of them being a standard. 

 The electromotive force thus produced was measured by noting the 

 double deflection on a galvanometer and then calibrating this galvanom- 

 eter deflection against a known electromotive force. 



Apparatus (Figure 4). — This apparatus consists of two cylindrical 

 sheet iron tanks, each 35 centimeters in diameter and 45 centimeters 

 deep, packed in a wooden box 45 X 50 X 100 centimeters. One hemi- 

 sphere was suspended on a wooden frame at the centre of each tank. 

 A fan at the bottom of each tank, which could be rotated in either 

 direction, and driven at various speeds by means of an electric motor, 

 served to keep the oil in the tanks in motion. 



The frames supporting the hemispheres extended into the tanks about 

 3 cm., thus preventing the oil from taking up a regular rotary motion. 

 These frames also served to hold the wires of the heating coil, by means 

 of which the temperature of the tanks could be raised. With a bank of 

 lamps and an adjustable resistance in series with the coils, the tanks 

 could be maintained at any desired temperature above that of the room. 



The two hemispheres were joined by a piece of the same lead as the 

 spheres. In order to insure perfect electrical contact, a small hole about 

 0.1 cm. in diameter was drilled into each hemisphere, to a depth of about 

 0.5 cm., and the end of the lead wire was carefully scraped and forced 

 into this hole with a wooden wedge. 



The leads from the nickel were brought to different points on the 



