B. A. Unit of Resistance. 235 



for 1+4, M=: 60257 at 17° 

 and " 2 + 3, M=101964 " 



Let p = ratio of the BA unit to the ohm, 



cib 



R= =" effective " resistance, 



a + o + c 



N=No. revolutions per second. 



D=D S — D N := difference of mean deflections, D s and D N , for 

 the two positions, S and N, of the reversing key ; i. e. D 

 would be the mean deflection for either direction of cur- 

 rent, if no irreversible effects existed. 



Then will 





and 



i R^-w^ - D rs y (!) 



! _l^D _T) i 



MN r3 1 r3 N 



f **1-8R --R 1 



- = ^r- i R 2 - #*- -D 2 )- (2) 



P MN 3 ] 2 N r3 2 f 



I N~ 8 13 J 



(1) is used when D 1 <^ D. 7 



(2) " " D^Dg. 



The double subscripts, as R 13 , means that the two quantities 

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

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

 sub 3's. It was found more convenient to calculate the values 

 of p t and p 3 this way and average them, than to apply an aver- 

 age correction. Indeed, when the speeds Nj and N 3 are differ- 

 ent, this is the only way. 



The following table gives the data and results of these ex- 

 periments; the (+) direction of rotation is, —Zenith, North, 

 Nadir, South. 



Experiments No's. 1, 2, 3, 6, 14A', 17A' and 22 were inter- 

 rupted by divers accidents and never completed; in No. 19, 

 there is confusion in the notes making the sign of the deflec- 

 tion doubtful ; Nos. 21, 23 and 24, give values of p from 2 to 

 10 per cent out, due to some error in the record of resistances 

 used. This accounts for all the experiments begun. 



The average of all the above is *98622 ; without No. 27, 

 which differs about twice as much from the mean as any other 

 observation, the average is -98634. The great divergence of 

 No. 27, is in itself reason enough for giving it less weight ; 

 but in addition, the chronograph sheet shows that the speed 

 here was very irregular, increasing, decreasing and increasing 



