412 ME. DUGrALD M‘KICHAN ON THE DETERMINATION OE THE NUMBER 
rays of light passing through the glass, and to avoid any unnecessary diminution of their 
brightness, the glass was removed and sheets of cardboard, with apertures through which 
the rays might pass, were substituted. Two or three such screens (l), their apertures 
varying in size with the distance of each screen from the coil, formed a sufficient pro- 
tection to the coil from currents of air. The box was large enough to admit of the 
free motion of the coil through a considerable angle; and for temporary adjustments 
the coil was accessible from behind, an aperture being left which was closed while obser- 
vations were being made, but which could be opened when necessary. 
The coil was not suspended from the roof of the box, which did not stand so high as 
the outside coils ; but a higher point of suspension was found by erecting a brass vertical 
tube ( d ), about 2 centims. in diameter and 35 centims. in length, over an aperture in the 
roof of the box. The suspending-wire was attached to the cover ( e ) which closed this 
tube. This cover or lid, which fitted into the mouth of the tube, being movable, the 
position of the coil in azimuth was easily adjustable. The suspending-wire, which acted 
also as a conducting-wire, was soldered to a cross piece fixed to the flanges of the ring at 
right angles to them : this wire, being thus in metallic connexion with the ring to 
which one end of the insulated wire was soldered as in the fixed coils, served as one of 
the terminals of the suspended coil. The other end of the insulated wire was soldered 
to a brass terminal set in a cross piece of vulcanite at the bottom of the coil ; to this 
terminal a long very fine platinum wire was attached, its other end being soldered 
to a stout wire fastened to the bottom of the box, and leading out to the other con- 
nexions. 
The movable coil was suspended symmetrically with respect to the fixed coils. Its 
centre was accurately marked on a piece of wood temporarily inserted inside the 
ring. The coil was then raised by means of the suspending-wire until this centre 
coincided with a fine wire temporarily fixed in the line joining the centres of the fixed 
coils. 
Again, by altering the position of the vertical tube relatively to the box to which it 
was fastened, the fixed coils being kept vertical, the vertical axis of the coil was adjusted 
to be equidistant from the planes of the fixed coils. 
The three coils just described were joined in series, the connexions being so arranged 
that a current passing through the three coils should pass in the same direction through 
the two fixed coils so that they should conspire in their action on the movable coil. 
The strength of the current was measured by the deflection of the suspended coil. 
In order that this determination, by means of the deflection, might be absolute, it was 
necessary to eliminate the effect due to the horizontal component of the earth’s mag- 
netism. This was done by observing the deflections when a given current passed first 
in the one direction and then in the other, the relations between the coils themselves 
remaining fixed. The mean deflection was that due to the simple action of the coils. 
To render the discrepance between the two deflections as small as possible, magnets 
were fixed in the neighbourhood of the coils to neutralize the action of terrestrial mag- 
