VERDET’S CONSTANT IN ABSOLUTE UNITS. 
3 
suspended near to it and outside it, and the rotation was measured by means of a divided 
circle. 
The methods by which the light was polarized and analyzed are described later. 
The investigation then resolved itself into three parts : — ■ 
(1) The determination of the constants of the helix. 
(2) The determination of the ratio which the rotation per unit of length bore to the 
tangent of the deflection of the suspended needle. 
(3) The determination of the horizontal component of the earth’s magnetism at the 
time and place of observation. 
The greater number of the experiments were made in my laboratory at Pixholme, 
near Dorking, but the determination of the number of windings and part of the deter- 
mination of the area of the helix were made in the Cavendish laboratory in Cambridge. 
DESCRIPTION OE INSTRUMENTS EMPLOYED. 
The helix was about 26 - 34 centims. long, about 5 centims. internal diameter, and 
12'5 centims. external, and contained about 35 lbs. of No. 20 cotton-covered copper 
wire. The insulation-resistance was exceedingly high, sufficiently so to enable me to 
neglect leakage. The resistance of the helix was 1*01 B.A. unit. 
The small dynamometer consisted of an ebonite ring, about 12 centims. diameter, on 
which were wound six turns of wire. It was made for me by Messrs. Elliott, and the 
mean diameter of the coils accurately gauged. 
The large dynamometer was the instrument belonging to the British Association. It 
is described and figured in Maxwell’s ‘ Electricity,’ vol. ii. fig. 54, p. 330. (The sus- 
pended coil there shown was not used.) 
The 'polarizing apparatus consisted of a Nicol’s prism, rather more than 1 centim. dia- 
meter, fixed into a collimating tube taken from a spectroscope. The slit was removed, 
and the Nicol substituted in its place. 
The analyzing apparatus. — This, together with its circle and the Nicol, is the property 
of the Cavendish laboratory, and was kindly lent to me by Professor Maxwell. The 
analyzer was a Jellett prism, described in the next paragraph. It was mounted in a tube, 
together with a lens and some diaphragms, which gave very good definition (see Plate 1). 
The Jellett prism* consisted of a piece of Iceland spar, of which the ends were cut 
normal to the sides. It was then divided by a plane, making an angle of about 1° with 
the plane containing the two long diagonals. One half was reversed, and the two 
cemented together again. 
The prism was mounted in a brass tube, and a diaphragm, with a hole of some 3 or 
4 millims. diameter in it, was placed across one end. On looking in from the other end 
of the prism, the hole was seen by the ordinary rays in the form of a circle divided by a 
* This prism is described by Prof. Jellett in the ‘ Transactions of the Royal Irish Academy,’ vol. xxv. Prof. 
Jellett also had the kindness to advise the author as to the form of his instrument best suited to this 
investigation. 
B 2 
