of the B.A. Unit of Resistance in Absolute Measure. 101 



To keep the temperature constant, spirals of lead pipe were 

 placed round the Elliot coil and in the inner cylinders of the 

 comparators, through which there was a constant flow of 

 water from the city supply. This answered its purpose ad- 

 mirably : the temperature varied only a degree or so even 

 from day to day. The water was of course allowed to flow 

 some hours before beginning observations. 



The terminals of all resistances were brought to large 

 mercury-cups, m, m, each having an amalgamated copper 

 disk lying on the bottom. The main current did not flow 

 through any part of the circuit of the induced current ; a 

 short bridging-piece, d, is used, as Lord Rayleigh found 

 necessary. 



The resistances used were all compared several times by 

 different observers with the standard : this was a Warden 

 Muirhead 10-ohm coil, whose value was determined at the 

 Cavendish Laboratory in 1887 ; it was 9*99416 B.A. units at 

 16°*5, with temperature-coefficient of "000292 per ohm per 

 degree. 



In taking the observations, the aim was to adjust the resist- 

 ances first so that there should be only a small deflexion. 

 After a number of galvanometer-readings for this "balanced" 

 arrangement had been taken, the resistance c was changed so 



o # t "... ^ 



as to give a deflexion of ten divisions (say) ; readings were 

 taken for this " unbalanced " arrangement ; the original 

 u balanced " was then restored and readings taken. If 

 nothing had changed sensibly since the beginning of the ex- 

 periment the average deflexions for the two " balanced " 

 would agree : of course this condition was only approximated 

 to. The " unbalanced" set gives the data for correcting for 

 the small deflexion of the " balanced." 



Each experiment then consists of the galvanometer-, speed-, 

 temperature-, &c. readings pertaining to the three arrange- 

 ments of resistances : these three arrangements are called A, 

 B, and A', in the order taken, irrespective of the magnitude 

 of the deflexions. In general Bj and B 3 (subscripts 1, 2, 3 

 refer to A, B, A x respectively; R is the "effective'"' resist- 

 ance) are the same, and the corresponding deflexions are small; 

 B is in this case used to correct both B x and B 3 , and the mean 

 of the corrected values is used. When, however, the deflexion 

 for A happens to be undesirably large after beginning the 

 experiment, B is made to give a small deflexion, and A! made 

 as nearly as may be the same as A. We have in this case to 

 apply the mean of two corrections to B 2 , one from A and the 

 other from A'. 



In each arrangement, as A, the current is reversed four 



