Properties , Specific Resistance, and Hardness of Steel. 351 



The Bunsen cups were introduced without altering the re- 

 maining parts of the circuit. The length of the wire heated 

 was not in all cases the same. 



B. The following determinations were made in the summer 

 of 1878 ; temperature of the room very constant, and at about 

 20°, this, as already observed, being nearly the same as t, the 

 temperature of the water in the colder receiver. The method 

 of compensation was employed throughout. The determina- 

 tion of T was effected by a Geissler normal thermometer (gra- 

 duated in 0°*1), that of t by an ordinary instrument (graduated 

 in o, 2), which had, however, been carefully compared with 

 the former. 



In Tables II. and III. the difference of temperature of the 

 ends of the steel rod is given under r, the corresponding elec- 

 tromotive force for the elements copper-steel under E T ; u and 

 ft are constants which satisfy the equation E t =ut—/3t 2 . 

 These were calculated by first computing their proximate 

 values out of two distant observations, and then adding to 

 them corrections deduced from five of the most satisfactory 

 observations by the method of least squares. 



If, now, we denote by a and b the constants for an ele- 

 ment of soft steel, hard steel corresponding to those a, /3 and 

 a', j3 f of the same rods when compared with copper, we shall 

 have, since E T =ar—i3r 2 and E' T =a!r — fi'r 2 , 



E T -E'=(*-*')T-(l3-j3>y. 



But i£ T — i? T =E T , the electromotive force of the element 

 steel-steel; so that, since also E T =ar— br 2 , 



a = cc — «', 

 b = /3-/3 ! . 



The constants a and b are given in the last two columns. 

 a may be regarded as numerically equal to the T. E. H. above 

 defined, as E T is nearly a linear function of t. 



