Mr. F. Jenkin on the New Unit of Electrical Resistance. 485 



surement, that he has been led to devise the following definitions, 

 in which none but already established measures are referred to. 



The resistance of the absolute is such that the current ge- 



second 



nerated in a circuit of that resistance by the electromotive force due 

 to a straight bar 1 metre long moving across a magnetic field of 

 unit intensity * perpendicularly to the lines of force and to its own 

 direction with a velocity of 1 metre per second, would, if doing no 

 other work or equivalent of work, develope in that circuit in one 

 second of time a total amount of heat equivalent to one absolute 

 unit of work — or sufficient heat, according to Dr. Joule's experiments, 

 to heat 0'0002405 gramme of water at its maximum density 1° Cen- 

 tigrade 



The new standard issued is as close an approximation as could be 

 obtained by the Committee to a resistance ten million times as great 



as the absolute . The straight bar moving as described above 



second 



in a magnetic field of unit intensity, would require to move with a 

 velocity of ten millions of metres per second to produce an electro- 

 motive force which would generate in a circuit of the resistance of 

 the new standard the same current as would be produced in the cir- 

 cuit of one — resistance by the electromotive force due to the 



second 



motion of the bar at a velocity of one metre per second. The velo- 

 city 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 therefore 



be said to be ten millions of metres per second, or 10 7 -. 



second 



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

 purpose 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 measured. 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 correla- 

 tive 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. 



* Gauss's definition. 



t 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. 



