4 
ME. J. E. H. GOEDON ON THE DETEEMINATION OE 
line across it, the light of the two semicircles being polarized in planes making with 
each other an angle of ahont 2°. 
The image of the hole formed by the extraordinary rays consisted of two semicircles, 
one to the right and the other to the left of this circle. 
A second diaphragm hid the latter. Now when light that has passed through a Nicol 
is examined with the prism, and the latter turned so that the light of one half of the 
circle is extinguished, the other half is slightly illuminated. If the Jellett be turned 
through rather less than two degrees, the second half of the circle will become dark, and 
the first will be slightly illuminated. 
It is obvious that there is a position between these two where the illumination over the 
whole circle is uniform, and this position can be observed with considerable accuracy. 
With the arrangements that were used in this work the probable error was about 1'. 
The divided circle was made for this investigation by Mr. Browning, and is about 
14 centims. diameter. The circle turns against two fixed verniers. It is divided on 
brass to 1°, and the verniers read to -ff, i. e. to o'. By estimation 1' can be read with 
perfect accuracy. It is moved by a screw of long pitch, working in a thread cut round 
its edge. This gives a sufficiently quick motion to enable changes in the illumination 
of any part of the field to be noted, and is yet capable of very delicate adjustment. 
The galvanometer consisted of a minute suspended magnet and mirror, weighing about 
1 grain, acted on by the helix (outside of which it was placed), and observed by a 
telescope and scale. 
The tube for containing the' bisulphide was of brass, with glass ends, and projected 
some 20 centims. from each end of the helix. 
The experimenting table was built of massive brickwork, with a carefully levelled top 
of fine cement. It was about 1-4 metre square. A continuation of it supported the 
suspended magnet, and a separate brick pier the telescope by which the magnet was 
observed (see Plate 1). 
The apparatus for measuring horizontal distances consisted of a number of conical 
plumb-bobs, which hung by silk threads from laths laid across a horizontal deal frame 
the same size as the table, and fixed about 1 metre above it. Distances projected on 
the table by these were taken by compasses, and measured on a millim. scale engraved 
on a massive brass plate by Elliott (for another purpose). 
The base of the circle, the telescopes, &c. were all placed on pillars of brickwork 
built on the tables, and bedded either in cement or plaster of Paris. The result was a 
steadiness such as I have never before obtained. 
THE EXPEEIMENTS. 
Determination of the Number of Windings. 
We must first determine the difference of magnetic potential at the two ends of the 
helix when a unit current passes through it. It is a quantity which we call N, and is a 
function of the number of windings and their arrangement ; for if we know the magnetic 
