Physical Units of Nature. 383 



would in that time change its velocity by one metre per 

 second; 

 ni x , the metric unit of the coefficient in the expression 



fi — y~ f° r * ne gravitation of two masses towards one 



another : this unit is the coefficient which should be 

 used if each gramme mass attracted other ponderable 

 matter at a metre distance with such intensity as would 

 impress on it an acceleration towards the attracting 

 gramme of one metre per second per second. 

 ei, the electromagnetic electrine, or the electromagnetic 

 unit quantity of electricity in the metric series, is that 

 quantity of each of the two kinds of electricity which 

 must be discharged every second in opposite directions 

 along a wire in order to maintain in it the metric unit 

 current, — this currentine or unit current being defined as 

 the current which must exist in a wire a metre long in 

 order that it may exert a force of a hyper-decigramme 

 on ponderable matter at a metre distance charged with a 

 unit of magnetism ; and the unit charge of magnetism of 

 either kind being defined as that quantity which, acting 

 on ponderable matter at a metre distance, charged with 

 an equal quantity of magnetism, exerts on it the unit 

 force — that is, one hyper-decigramme. 



3. It is easy to ascertain the relation of this metric electrine 

 to the B.A. (British-Association) standards for electrical mea- 

 surement, which are those most in use. The B.A. units are 

 electromagnetic units based on the following fundamental 

 units — the second for unit of time, the metre-seven (the quad- 

 rant of the earth, or 10 7 metres) for unit of length, and the 

 eleventh-gramme (or gramme divided by 10 11 ) for unit of 

 mass. These were so chosen as to furnish a connected body 

 of systematic units with such values that the practical electri- 

 cian could conveniently use them. Now the u dimension " of 

 electromagnetic quantity of electricity is [L\/LM] (see B.A. 

 Report for 1863, p. 159)*. Hence, and from the foregoing 

 values of the lengthine and massine of the B.A. series, 



e 1 : one Ampere = 1 : ^/^x 5 

 therefore ^ = 100 Amperes. 



• This follows at once from the fundamental equations of electromag- 

 netism, viz.: — 



EE 1 . ™ «, *, Cm -, MM 1 



Fa 



Ct; F=^ ; F=^, 



