STRAIN ON THK PHYSICAL PROI'KUTIKS OF MATTER. 



23 



Experiment XIX. 

 IRON wire 365 centimetres in length and '07035 centimetre in diameter. 



Experiment XX. 

 PIANO-STEEL 365 centimetres in length and '0824 centimetre in diameter. 



The principal difficulty connected with the accurate reduction of these experiments 

 lies in the calculation of the rise of temperature produced by the current. According 

 to J. T. BOTTOMLEY * the rise of temperature may be obtained from the following 

 easily proved formula : 



where t is the temperature of the wire, 6 the temperature of the room, c the current, 

 <r t the specific resistance of the wire at the temperature t, J JOULE'S equivalent, d 

 the diameter of the wire, and e the emissivity of the wire. It would follow from 

 Mr. BOTTOMLEY'S experiments that with wires whose diameters vary from 0'085 centi- 

 metre to 0'040 centimetre the value of e ranges from ToW to 4^5, or approximately as 

 the inverse of the square of the diameter. On this last assumption the values of e 

 for the iron and steel wires were calculated to be TjW an< i 16*79 respectively. The 

 values of ov were calculated from a knowledge of the specific resistance of the two 

 metals at the temperature of the room, which was 18 C., and of the coefficient of 

 increase of resistance per degree rise of temperature. The specific resistances of the 

 iron and steel wires at the temperature of 18 C. were 9707 and 20,742 respectively, 



' Nature,' September 25 and October 2, 1884. 



