424 



Dr. S. Bidwell. On the Changes of 



[Apr. 11, 



thermoelectric force produced by the application of the corresponding 

 external field H. Curve (A) relates to a specimen of iron wire 



'J 



















1 



















)Ll- _ 





(G) 

















X) 6< 



>Q 8< 



SO TO 



DO 12 



^Magnetic 





















4a 



ijj 



40; 



60 



Fig-. 3. — Curves (A), (B), and (C) show changes of thermoelectric force for pur& 

 iron, commercial iron, and steel respectively. Curve (a) shows corresponding 

 changes of length for iron. 



0-28 mm. in diameter, supplied by Messrs. J. J. Griffin and Sons as 

 pure. Details of the experiment are given as an example of the 

 manner of working : — 



Method II. — E - 14,000; r = 10 ohms. 

 Coil No. 1. — H = amperes x 92. 

 Temperature of cold water, 14° C. 



Resistance of galvanometer 4*70 ohms. 



„ leading wires 0*39 „ 



„ potential coil 10*0 „ 



„ thermo-rocl 0*20 „ 



Total resistance in circuit 



15-29 



Electromotive force of 1 microvolt gives 1/15-29 microamperes = 

 54/15-29 scale divisions, or deflection in scale divisions x (15*29/54 =} 

 0*283 = microvolts. 



Before every observation the iron was demagnetised by Ewing's 

 method of reversals, alternating currents gradually decreasing to zero 

 being switched into the magnetising coil. 



The difference of the temperatures of the water at 14° and of the 

 steam at 100° was 86° ; assuming that the actual temperatures of 

 the junctions were respectively \° above and below those of their 

 surroundings, the change of thermoelectric power for a given field 



