YERDET’S CONSTANT IN ABSOLUTE UNITS. 
17 
sewing-needle was set upright in a hole drilled in the table, and filled with soft clay, 
at such a height that the magnetic needle when balanced on it just swung clear. 
The latter was allowed to come to rest under a bell-glass, and then clamped by a 
simple arrangement. As the ends of the straw were 5 or 6 millims. from the table, a 
candle was placed close to the centre of the needle, and the ends of the shadow of the 
straw marked with a pencil. This meridian was then transferred to any other part of 
the table by means of parallel rulers *. 
The following method was used for placing the helix at a known distance from the 
suspended magnet and perpendicular to the magnetic meridian, and for making that 
meridian which passed through the suspended magnet bisect the axis of the helix : — 
A pencil line longer than the helix was drawn through the point on the meridian 
selected for the centre of the helix and perpendicular to that meridian. The plumb- 
bobs were hung at points, one near each end of this line and in it, and two more in the 
meridian passing through the suspended magnet. 
A mark was made at the centre on the outside of the helix, and cross-wires placed at 
its ends inside. When the threads of the E.W. plumb-bobs sighted the cross-wires and 
the meridian-threads the point on its outside, the helix was properly placed. 
Verification of the Meridian. 
In order to see that there had been no error, the following verification was made, 
based on the principle that if the helix was placed symmetrically with regard to the 
meridian, the deflections produced in opposite directions by reversing the same current 
should be equal. 
Readings. 
Current direct. 
No current. 
Current reversed. 
8-47 
42-17 
8-54 
25-48 
42-11 
Corrected Deflections. 
16-84 . . . 16-64 
A difference too small to be worth correctingf. 
The Light. 
It was absolutely necessary that the light should be monochromatic, as the rotation 
of different colours for the same difference of magnetic potential is very different, varying 
* See verification of meridian below. 
t Professor Maxwell has since pointed out to me that a slight displacement of the helix would not make so 
large a difference in the deflections as I had expected. I, however, leave in this paragraph for what it is worth. 
MDCCCLXXVII. D 
