234 



Dr. J. Monckman. Occluded Gases and [May 31, 



Changes per 1° C. per 1 ohm — 



Expt. 1 gives— 21 }0 . 0009 Expt. 2 gives- 22 }Q mQm 



l^ho -00068 l^ho -000678 



*'°}0 -00070 f 2 }0 -00038 



^}0 -00076 '00052 



Expt. 3 gives — '00031 Expt. 4 gives 22 



226 

 257 

 298 

 347 



}0 -00025 

 }0 -000195 

 }0 -00032 



221 



273 

 325 



}0 -00031 

 }0 -00026 

 }0 -00030 



All showing a decrease (in the temperature coefficient) to about 

 250°, and then an increase. 



This method cannot lay claim to absolute accuracy, as there is in some 

 cases an increase of resistance by the change in the contact of copper 

 with carbon, which appears when the rod cools as in Experiment 2. 

 This, however, takes place at the higher temperatures, and tends to 

 decrease the numbers obtained at those temperatures, and a correction, 

 if one could be applied, would only increase the results obtained in 

 the previous experiments. 



Coefficient of Expansion. 



Method. — As we wished to raise the rod to 500° or 600° C, it was 

 impossible to expose the whole rod to that temperature, and at the 

 same time to read the changes of position of a mark or point at the 

 end of it with a microscope ; nor did it appear probable that contact 

 could be made by rods of other materials. 



It was decided, therefore, to heat the central portion of a rod, 

 keeping the end portions cold. We had thus one hot portion, two 

 colder, and two others at a constant temperature. A rod, about 

 36 inches in length and J inch in diameter, was ujed. One end was 

 electroplated and then soldered into a cavity in a brass rod which 

 was firmly clamped to a vertical iron one fixed to a stone table. Into 

 a small hole in the other end a fine needle was fixed whose change of 

 position was read by a microscope. 



The central portion of the carbon was covered with a thin coating 

 of clay, then with paper to consume the oxygen, outside that a glass 

 tube packed with asbestos inside of a porcelain tube. 



Ten inches of the centre of this was heated in a gas furnace. The 

 temperature was taken with a platinum thermometer (fig. 5), EF 

 giving the temperature of the hottest part, AB and CD those of the 

 portions between the hottest and the constantly cold portion. 



