VEBDET’S CONSTANT IN ABSOLUTE UNITS. 
5 
intensity at eacli point of the axis of the helix due to a unit current, viz. what force is 
exerted by a unit current in the helix on a unit magnetic pole at that point, then we 
know how much work would have to be done to move this unit pole from one end of 
the helix to the other when a unit current was passing in it. 
But if a , b be the ends and u x the force at any point, then the above amount of work 
would be equal to 
j \ x dx=V b —Y„ (1) 
where V is the potential at any point. 
But the dimensions of magnetic potential are, in the electromagnetic system, 
- [LWT- 1 ]; 
and these are also the dimensions of the strength of an electric current. 
.’. N, which is the ratio of the former of these things to the latter, is a number. 
The value of N for the helix was determined by comparison with the great dyna- 
mometer of the British Association, which is deposited in the Cavendish Laboratory. 
The intensities of the magnetic action were compared at 7 equidistant points in the 
axis of the helix, and the total force was obtained by integrating by Weddle’s formula 
(Boole’s ‘Finite Differences,’ p. 47), viz. 
\ u x dx=$ s h{ w 0 +M 2 +w 4 +?i 6 +5(?ii+« s ) + 6m 3 [*, (2) 
Jo 
where h is the distance between any two of the points, and u x is the magnetic intensity 
at any point in terms of that of the dynamometer. 
The mechanical arrangements were as follows : — the dynamometer, which consists of 
two coils about 50 centimetres in diameter, at a distance of about 25 centimetres one 
from the other, was placed so that the plane of the coils was accurately vertical and in 
the magnetic meridian. 
The axial line of the coils was then carefully found and marked by means of plumb- 
lines and cross-threads fixed to the table. 
A strong T-shaped board supported on three levelling-screws was placed so that the 
part corresponding to the stem of the T passed through the coils in a horizontal plane 
parallel to their axis ; on this the helix was laid and its axis brought into exact coinci- 
dence with that of the coils. 
A boxwood cylinder about 20 centims. long was turned to fit nicely into the helix ; a 
long thick brass wire terminating in a handle was fixed into one end, and a brass pin 
about 5 centims. long was fixed-near one edge of the other. 
* Objections have been taken to the use of this formula, which gives, it is observed, much more weight to 
and u 5 than to u 2 and u 4 , and does not furnish an approximation of a legitimate analytical character. While 
fully acknowledging the force of these objections, I have not thought it worth while to make an alteration which 
would involve repeating most of the arithmetic in the paper, for this reason : the experiments, being made with 
resistance-coils, are susceptible of such close accuracy that the errors of any particular determination, even when 
multiplied by 5, cannot perceptibly affect the value of N deduced. 
