BISHOP. — THERMAL CONDUCTIVITY OF LEAD. 



683 



connected in series through a resistance, and the drop of potential across 

 this resistance used as the source of electromotive force. This was 

 counterbalanced against a Carhart-Clark cell, which had been recently 

 calibrated. 



9. Sensitiveness of the galranometer used to balance the electromotive 

 force against the Clark cell. — This was of sufficient sensitiveness to 

 detect a change of 2 ohms in 10,000 ohms in the Carhart-Clark cell 

 circuit. 



10. Insulation. — The galvanometer leads were carried through air to 

 the stone pier upon which the galvanometer rested. The leads from the 



50 



40 



o 30 



2 20 



2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 



Time in Minutes. 



Figure 6. 

 Comparison of variation (vide error 11). 



heating coil were brought out through the box in glass tubes. The 

 nickel wires were carefully insulated both inside the tanks and out, and 

 were carefully separated even when entering the mercury cups. 



1 1 . Variation in temperature during a reading. — Since it was im- 

 possible to maintain the tanks at an absolutely constant temperature, it 

 was important to know how closely the galvanometer deflections varied 

 with the variations in the readings of the thermometers. In order to 

 show this, a special set of readings was taken with large variations, and 

 the following plot (Figure 6) shows the results. The results for the 

 plot were obtained by taking the difference in temperature, and the gal- 



