HALL. — THERMAL AND ELECTRICAL EFFECTS IN SOFT IRON. 45 



ticular, that they would leave an error of about one sixth of one per cent 

 in the estimation of the mean temperature of the bars in which the 

 Thomson effect occurs. 



Further and more exacting uses of these couples, or others similar, are 

 in view, however ; and the study of their behavior will be continued. 

 See p. 54. 



Resistances. 



The resistances used in this investigation were, for the most part, in 

 the form of detached coils of manganine, made and tested by the methods 

 which Professor P. O. Peirce has established at the Jefferson Laboratory. 

 The greatest proportional error found in any of these coils when they 

 were tested at the end of the work, in December, 1904, was about oue 

 third of one per cent in a certain 10 ohm coil, which proved not to be 

 made of the best wire. Error from the use of this coil was probably not 

 more than one seventh of one per cent in any important place, and this 

 error has been disregarded in the calculations. 



The resistances of the thermo-electric couples and of the connecting 

 wires of the various circuits were usually measured by means of a well- 

 tested Wheatstone bridge, in box form. 



Current Measurers. 



On and after August 31, 1904, the instrument used for the measure- 

 ment of thermo-electric currents was a three-cased Panzer-galvanometer of 

 the du Bois and Rubens design. This was entirely satisfactory in regard 

 to sensitiveness and freedom from external magnetic disturbances; but 

 on the other hand its sensitiveness, when great, was subject to large and 

 quick changes, sometimes amounting to several per cent in a single hour, 

 and accordingly it was necessary to make frequent measurements of its 

 reduction factor. For this purpose a potentiometer method was used, 

 with a cadmium cell as a standard of electromotive force and with a num- 

 ber of well-tested resistances. Compared, about the middle of November, 

 1904, with a number of other cells in the care of Professor B. O. Peirce, 

 the cell here used appeared to have an electromotive force of 1.0178 

 volt. Tested more carefully in March, 1905, under the direct super- 

 vision of Professor Peirce, it showed, near 1G° C, an electromotive of 

 1.0184 volt. 



The galvanometer coils were marked by the manufacturer as having 

 5.5 ohms and 5.3 ohms resistance, respectively, at 20° C, and this cer- 

 tificate was found to be correct. 



