646 
MR. CLERK MAXWELL ON A DIRECT COMPARISON 
If we take into account the fact that the section of each coil is of sensible area, this 
formula would require correction ; but in these coils the depth was made equal to the 
breadth of the section, whence it follows, by the differential equation of the potential of 
two coils, given at p. 508 of my paper on the Electromagnetic Field, 
d*M ldM 
dc? db 2 'a da ’ 
that the correction is a factor of the form ^1 — - ^ , where a! is the depth of the coil 
— a correction which is in this case about 1 — *000926. 
The suspended coil, besides the repulsion due to the fixed coil, experiences a couple 
due to the action of terrestrial magnetism. To balance this couple, a coil exactly similar 
was attached to the other arm of the torsion-balance, and the current in the second coil 
was made to flow in the opposite direction to that in the first. When the current was 
made to flow through both coils, no effect of terrestrial magnetism could be observed. 
A. Suspended disk and coil. 
A'. Counterpoise disk and coil. 
C. Fixed disk and coil. 
B 2 . Great battery. B 2 . Small battery. 
G, . Primary coil of galvanometer. G 2 . Secondary coil. 
R. Great resistance. S. Shunt. 
K. Double key. g. Graduated glass scale. 
C. Electrode of fixed disk. 
x. Current through R. 
x'. Current through G r x—x'. Current through S. 
y. Current through the three coils and G 2 . 
M. Mercury cup. T. Torsion head and tangent screw. 
One quarter of the micrometer-hox, disks, and coils is cut away to show the interior. The case of the instru- 
ment is not shown. The galvanometer and shunts were 10 feet from the Electric Balance. 
