ROTATION OF LIGHT IN BISULPHIDE OF CARBON. 
349 
length. The operation was performed on December 14-15, 1883, with triply-covered 
wire of diameter about inch, and no particular difficulty was experienced. The 
revolutions of the ebonite tube, mounted in the lathe, were taken with all care by an 
engine counter, and amounted to 1842, so that the total number of windings is 3684. 
The internal diameter of the helix is 2’188 inches, and the external diameter is 
4T3 inches. 
By endeavouring to force a current from one wire to the other we obtain a very 
severe, though of course not absolutely complete, test of the insulation. The resistance 
between the two wires varied with the hygrometric condition of the silk, which was 
not impregnated with paraffin. At first it was not much over 2 megohms, but 
latterly reached 6 or 8 megohms, and was thus abundantly sufficient. 
14. As a further test observations were made of the external effect of the helix 
upon a suspended magnet, when a powerful current was passed in one direction 
through the first wire, and in the opposite direction through the second. If the 
positions of the two wires could be treated as identical, the external effect ought 
everywhere to vanish. In consequence, however, of the fact that one wire lies 
throughout on the same side of the other, the compensation could not be expected to 
be complete, except when the suspended magnet is equidistant from the two ends. 
Experiment with the magnet of a reflecting galvanometer showed that the effect, in 
fact, varied as the magnet was displaced, but even in the symmetrical position there 
was a perceptible outstanding differential effect. In order to eliminate the influence 
of other parts of the circuit, the readings referred only to the deflection of the needle 
as the current was reversed in the helix; and the scale of sensitiveness was obtained 
by repeating the observations after altering the connexions of the two wires, so that 
the current circulated the same way round both, and after insertion of a high resis¬ 
tance by which the intensity of the current was reduced in a known proportion. 
From this it appeared that the differential effect of the two wires (with a given 
current) was y so o °f the combined effect. 
This fraction is tolerably small, but I had expected to find it smaller still. It seems 
probable that the incompleteness of compensation is due to a small difference ( 5 W 00 ) 
in the mean diameter of the windings in the two cases. To throw light upon this 
I took careful measures of the resistances of the two wires. Although they had 
originally formed one length, their resistances differed by as much as - 7 \jth part, that 
of the wire which had shown itself least effective being 7'075 B.A., and of the other 
6 965. If, as it seems plausible to do, we attribute the difference of resistance to 
difference of diameter, this actual difference must amount to -^sVo inch. The mean 
diameter of the windings is about three inches ; and if the two wires were wound upon 
a smooth cylinder of this diameter, the difference in the diameter of the windings 
would be -g-^o of the whole. As this estimate would be increased were we to take 
into account the fact that each winding really sits upon two windings of the layer 
underneath, and that these cannot be practically in actual contact, we may perhaps 
MDCCCLXXXV. 2 Z 
