488 REPORT— 1867. 



Independent series of observations divided into triplets : — 



first two, 0' 22|, 1' 24", last two, 9' 37|, 10"-39, 

 30 oscillations in all. Mean value of 2<=20-55. 



Value of li^. Direct method. Battery resistance : — 



r 22.3 



2° 216 



Mean 219 



Direct deflection : — 



d. I: B. ff. V. Ej Ohm's. 



1°....278 22000 219 47 1000 649 x 10" 

 2°....321i 19000 219 47 1000 649x10^ 

 Mean value of R^ in absolute units 6-19 X 10'". Value of S= 99-2 X 10 - '-. 

 October 17. — Discharge : — 



1°. 2°. 3°. 4°. Mean. 



179 180 179 180 179-5 



sin I { = 91 1 . 

 Times : — 



0'55", 1'56|", 10' 7i", 11' 8|". 



Total number of oscillations 30. Mean value of 2<:=20-4G. 

 Value of R . Direct method. Battery resistance : — 



P 210 



2° 22.3 



Mean 215'5 



Direct deflection. : — 



d. I. B. q. or. R^ Ohm's. 



1°. . . .268 22000 215i 47 1000 597 X 10^ 

 2°..... 329 18000 215i 47 1000 6-01 x lO'^ 

 Mean value of E^ = 5-99 X 10" absolute units. Value of 8 = 99-25. 



The seven values obtained for S give a mean value of '9965 X 10-^^ as 

 the capacity of the mica-plate condenser -when charged for one minute, and 

 measured by a discharge through a galvanometer, on the needle of which it 

 acts for about 5 seconds. If we reject the two observations made on Oct. 15, 

 which ■were, indeed, only preliminary, and made with less care than all the 

 others, we find the average to be 0-99G2 X 10 "1*^ and the approximation be- 

 tween this mean and any single results is 0-42 per cent. It is therefore 

 probable that a unit copied from this preliminary standard will not be one 

 per cent, wrong. 



A tenfold multiple (10"" absolute measure) of the condenser measured is 

 a convenient magnitude as a practical xmii of capacity for telegraphy ; thus 

 the capacity of the Atlantic cable per knot thus measured is 0-3535. Assum- 

 ing that the practical unit of electromotive force will be chosen as that mul- 

 tiple which is most nearly equal to Daniell's cell, i. e. 10" electromagnetic 

 iinits, then the capacity of the proposed practical iinit is such that it contains 

 with the imit E M F the same quantity of electricity as would be passed 

 in one second through a circuit of the resistance of one Megohm. Thus 

 105 E M F, acting on a circuit of lO^-^ ^jn p^jgg \^ qj^^ second 10— ^ absolute 

 units of quantity; and similarly, 10^ E M E will charge a condenser of ab- 

 solute capacity equal to lO" ^"' with 10—^ absolute units of quantity. This 

 practical series of units is that which, in the opinion of Mr. Latimer Clark 

 and myself, is best adapted for practical use in telegraphy. Mr. Clark caUs 

 the unit of quantity thus defined (10~^) one Earad, and similarly says that 

 the unit of capacity has a capacity of one Earad, it being rinderstood that this 

 is the capacity when charged with unit electromotive force (10^). 



