ON THE ABSOLUTE UNIT OF ELECTRICAL EESISTANCE 173 



seconds, but the time of vibration was too short and another needle was 

 constructed vibrating in 11-5 seconds, which was a sufficiently long 

 period to be used successfully after practice. 



There seems to be no error introduced by the time taken to reverse 

 the commutator in the method of recoil, seeing that the breaking of 

 the current stops the needle and the making starts it in the opposite 

 direction. As the time was only a fraction of a second the error is 

 minute in any case. 



While the current is broken in the reversal, the battery may re- 

 cuperate a little and there is also some action from the extra current, 

 but there seems to be no doubt that long before the four or six seconds 

 which the needle takes to reach its greatest elongation everything has 

 again settled to its normal condition and the current resumes its 

 original strength. Hence the error from these sources may be con- 

 sidered as vanishingly small. 



Some experiments were made by simply breaking the current and 

 they gave the same result as by reversal. 



The following is the order of observations corresponding to each 

 experiment. 



1st. The time of vibration of needle was observed. 



2d. The current was passed around the circle, F, so as to observe 

 y3 and a. Simultaneous readings were taken at the two galvanometers. 

 The commutator at the tangent galvanometer was then reversed and 

 readings again taken. After that the commutator to the circle was 

 reversed and the operation repeated. This gave four readings for the 

 circle and eight for the tangent galvanometer, as both ends of the 

 needle were read. In some cases these were increased to six and twelve 

 respectively. This operation was repeated three times with currents 

 of different strengths, constituting three observations each of a and /?. 

 To eliminate any action due to the induction coils, they were sometimes 

 connected in one way and sometimes in the opposite way. 



3d. The resistance of the circuit was adjusted equal to the arbitrary 

 standard. 



4th. The circle, F, was thrown out of the circuit and the observations 

 of 6 and d begun. Two throws, d, one on either side of zero were 

 observed and one reading of d taken. The commutators at s and C 

 were then reversed, and the operation repeated. This whole operation 

 was then repeated with currents of three different strengths. The 

 position of the two induction coils was now reversed and observations 

 again made with the three currents. The resistance was now com- 



