1865.] of Electrical Resistance. 163 



current as would be produced in the circuit of one j resistance by the 



second ' 



electromotive force due to the motion of the bar at a velocity of one metre 

 per second. The velocity required to produce this particular current* 

 being in each case proportional to the resistance of the circuit, may be used 

 to measure that resistance, and the resistance of the B. A. unit may there- 

 fore be said to be ten millions of metres per second, or 10 7 me re ^. 



second 



It is feared that these statements are still too complex to fulfil the pur- 

 pose of popular definitions, but they may serve at least to show how a real 

 velocity may be used to measure a resistance by using the velocity with 

 which, under certain circumstances, part of a circuit must be made to move 

 in order to induce a given current in a circuit of the resistance to be mea- 

 sured. That current in the absolute system is the unit current, and the 

 work done by that unit current in the unit of time is equal to the resistance 

 of the circuit, as results from the first equation stated above. 



Those who from this slight sketch may desire to know more of the 

 subject will find full information in the Reports of the Committee to the 

 British Association in 1862, 1863, and 1864. The Committee continue to 

 act with the view of establishing and issuing the correlative units of 

 current, electromotive force, quantity, and capacity, the standard apparatus 

 for which will, it is proposed, be deposited at Kew along with the ten 

 standards of resistance already constructed with the funds voted by the 

 Royal Society. 



APPENDIX B. 



The following Table shows the degree of concordance obtained in the 

 separate experiments used to determine the unit. The determinations were 

 made by observing the deflections of a certain magnet when a coil revolved 

 at a given speed, first in one direction, and then in the opposite direction. 

 The first column shows the speed in each experiment ; the second shows 



the value of the B. A. unit in terms of 10 7 meires as calculated from the 



second 



single experiments. A difference constantly in one direction may be 

 observed in the values obtained when the coil revolved different ways. 

 This difference depended on a slight bias of the suspending thread in 

 one direction. The third column shows the value of the B. A. unit 

 calculated from the pair of experiments. The fourth shows the error 

 of the pair from the mean value finally adopted. In the final mean 

 adopted, the 1864 determination was allowed five times the weight 

 allowed to that of 1863. 



* This current is the unit current, and, if doing no other work or equivalent of work, 

 would develope, in a circuit of the resistance of the B. A unit, heat equivalent to ten 

 millions of units of work, or enough to raise the temperature of 2405 grammes of 

 water at its maximum density 1 Centigrade. 



