348 
LORD RAYLEIGH OH THE CONSTANT OF MAGNETIC 
Series I., the apparent magnitude of the field was much less, and an opera-g-lass, 
magnifying about twice, was employed between the analyser and the eye. 
11. The setting of the Nicol (or double-image prism) by adjustment of the match 
between the two parts of the field presented by the half-shade apparatus was 
facilitated by a device that may be found useful. “ In addition to the principal helix, 
the tube was embraced by an auxiliary coil of insulated wire, through which could be 
led the current from a Leclanche cell. This current was controlled by a reversing 
key under the hand of the observer, who was thus able to rock the plane of 
polarisation backwards and forwards through a small angle about its normal position. 
The amount of the rocking being suitably chosen, the comparison of the three 
appearances (two with auxiliary current, and one without) serves to exclude some 
imperfect matches that might otherwise have been allowed to pass.” * 
12. Apart from the effect of heat upon the CS 3 , the working of the optical parts was 
fairly satisfactory. The following zero readings taken without the current on .June 4, 
1884, will give an idea of the sort of accuracy attained. The analyser was a double 
image prism, and was read in all four positions, the circuit being made three times. 
Table I. 
o / 
193 4 
o / 
o / 
13 2 
103 2 
283 0 
102 55 
193 5 
283 2 
12 59 
102 58 
193 3 
283 2 
13 4 
Mean. 
Subtract .... 
102 58 
90 
193 4 
180 
283 1 
270 
13 2 
12 58 
13 4 
13 1 
13 2 
It appears that an error of 3 or 4 minutes may occur in a single setting. 
13. I now pass to the description of the electrical arrangements. The magnetic 
force depends upon the helix and upon the strength of the current, and we will take 
these elements in order. 
The helix. 
The wire is wound upon an ebonite tube, the outside surface of which was turned 
true in the lathe, and is kept in its place laterally by ebonite flanges screwed upon 
the tube. The distance between the flanges, equal to the length of the helix, is 
9‘99-0 inches ; but the tube itself projects some inches beyond the flanges, and when 
it was desired to use an internal water-jacket, could be further prolonged by additional 
lengths of brass tube. 
In order to give better opportunity for testing the insulation, on which the correct¬ 
ness of the final results is entirely dependent, it was decided to wind on two wires 
simultaneously, which should be in contact with one another throughout their entire 
* “ Preliminary Note on the Constant of Electro-magnetic Rotation of Light in Bisulphide of Carbon.” 
Proc. Roy. Soc., vol. 37, p. 146 (June 19, 1884). 
