496 Sir Oliver Lodge on the 



& 



kept flowing, so as to screen the core from elevation o£ tempe- 

 rature due to the current in the exciting coil. The coil of wire 

 wound on each bobbin had a resistance of 31 ohms and con- 

 sisted of seven thousand turns of silk-covered No. 18 wire 

 ('048 inch diameter), wound in 22 layers, with 7'2 turns to the 

 centimetre in each layer ; thus giving a iC gauss-gradient," or 

 magnetic intensity, along the axis, of 200 times the amperes 

 flowing in the wire. 



With 111) volts applied, the current was 3J amperes ; with 

 230 volts applied, which could be done by putting the town 

 mains (at that time 110 volts) in series with a large storage- 

 battery, the current w r as 7 amperes, measured by an ampere- 

 meter in both instances. 



The corresponding intensities of field are 



H= *! i aqq c.G.s. lines to the square centimetre respectively. 



17. To check this estimate, the four bobbins were filled 

 with bisulphide of carbon, placed all in a row, and supplied 

 each with a current from the 110- volt mains, the coils being 

 electrically in parallel. Polarized sodium light was then 

 passed through the set, and the plane of polarization was 

 found to be rotated by the current 83° ; of which 5° was due 

 to the glass, and 78° to the bisulphide of carbon. 



Hence the drop of magnetic potential between the ends 

 of each coil comes out 29,000 gauss ; according to Lord 

 Eayleigh's estimate of the Verdet's constant for (JS 2 a ^ 18°, 

 viz., '042 minute of arc for every G.G.s. unit of magnetic 

 potential. The other estimate, from current and number of 

 turns, under the same conditions, made the drop of magnetic 

 potential 44x670 = 29,500 c.G.s.; which therefore agrees 

 quite sufficiently well for the purpose. 



18. All this is preliminary. In the actual experiment for 

 trying the accelerating effect of a magnetic field on light, the 

 bobbins were not placed in a row, but were arranged round 

 the sides of a square inscribed at 45° to the sides of another 

 square or optical frame (about 1 metre in the side) of which 

 the details are given in the Phil. Trans. 1893, pp. 757 and 

 761, or more exactly in the plate 31 at the end of that 

 volume (vol. 184) of the Phil. Trans. ; since it was the 

 same optical frame as was used for my aether convection 

 experiments therein described. (See also Preston's ' Light/ 

 § 136, and Proc. Roy. Inst. April 1, 1892.) 



It is sufficient to say here that a parallel beam of light, 

 bifurcated by semitransparent silvered glass at 45°, could be 

 sent, by carefully adjusted mirrors, several times round the 



