( 184 ) 



Although this construction (fig. 1) (for which ca block of greater 

 thickness is required than for that of Comm. N". 89, PI. I) has not 

 been applied to the element used, we need not fear uncertainties on 

 this point thanks to the very careful construction of tlie latter. 



b. When temperatures below — 253" liave to be determined we 

 might fdl the apparatus with helium instead of hydrogen as men- 

 tioned in § 1 of Comm. N". 89. 



c. The glass tubes of the mercury commutators, described in 

 Comm. N". 27, are not fixed in corks (see PI. IV, fig. 4, Jc) but in 

 parafifm, so as to obtain perfect insulation, which, as experience has 

 taught, is not guaranteed by the glass wall. Tlie tubes are continued 

 beyond the sealing places of the platinum wires c^ c^ Cjand c^, (as shown 



Fig. 2 and 3. 



by figs. 2 and 3) to avoid breaking of the platinum wires as 

 formerly frequently happened. 



d. The platinum wires of the Weston-elements have been ainal- 

 gamized by boiling with mercury (which method has since that time 

 been replaced by the method with the electric current ^)). The elements 

 themselves have kept good through all these years. 



e. In spite of all the precautions which have been described in 

 Comm. W. 89, thermo-electromotive forces still remain in the wires, 

 which with the great differences of temperature between various 

 points of one wire must doubtlessly amount to a measurable quantity. 

 When, however, care is taken that the circumstances under which 

 the element is used with respect to the temperature along the wires 

 are about the same as for the calibration, a definite value of the 

 electromotive forces will answer to a definite temperature of the 

 copper block. We do not aim at an accurate determination of the 

 electromotive force of the combination of the metals which at the 



1) Comp. Jaeger, Die Normalelemente, p. 57. 



