of the B.A. Unit of Resistance in Absolute Measure. 103 



The corrections, calculated by these formulae, due to the 

 change in a give 



for 1 + 4, M= 60257 at 17° C, 



for 2 + 3, M= 101964 „ 



Let p = ratio of the B.A. unit to the ohm, 



R= ; ''effective" resistance, 



a + b + c 



N= number of revolutions per second, 



D= D s — D N = difference of mean deflexions, 



for the two positions S and 1ST of the reversing-key ; i. e. D 

 would be the mean deflexion for either direction of current, 

 if no irreversible effects existed. 

 Then will 



1^2 — 1^1,3 



D.,s>, . . (1) 





\ Tfr'Dr-B,. 



and 



1 !^ N 2 



Ro-R 



1,3 



%- tf 1 



— U 2 — Ui,3 



(1) is used when T> 1 <D 2 ; (2) is used when D 1 >D 2 . 



The double subscripts, as R 1? 3 , means that the two quantities 

 R x and R 3 are to be used in turn ; that is to say, each formula 

 above is really double : first we use the subscript l's, and 

 then the subscript 3's. It was found more convenient to cal- 

 culate the values of pi and p 3 this way and average them than 

 to apply an average correction. Indeed, when the speeds Nj 

 and N 3 are different, this is the only way. 



The following table gives the data and results of these 

 experiments ; the ( + ) direction of rotation is zenith, north, 

 nadir, south. 



