Dispersion in Relation to the Electron Theory. 245 



consisting of 3750 turns of No. 18 double cotton-covered 

 copper wire in 12 layers, is wound in the space C between 

 the fibre cheeks, each layer being carefully insulated with 

 shellac (which was then dried by passing a current). The 

 ends of alternate layers are brought out to separate terminals 



Fig. 3. 



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rXKS^^vxm^^^ryr>n^ 



- 



**$** 





:^~ 



g> Ok. fe. 



on the ebonite uprights. The finished coil is 11 cm. external 

 diameter and 50 cm. long. The tube B carries an inner one 

 of 6 cm. internal diameter, the space between the two being 

 closed by rings at the ends, so forming a space for circula- 

 tion of the cooling-water. A thin brass rod bent into a 

 helix S is fitted between the tubes to give a spiral motion to 

 the flow of water. The tube to contain the liquid under 

 examination, A, is mounted in a wider tube T forming a water- 

 jacket, the latter being held in position in the inner brass 

 tube by means of brass collars provided with spring clips. 

 The water-jacket carries also two thermometers, and a spiral 

 of No. 24 constantan. By means of the latter coil, which 

 was connected to an accumulator, amperemeter, and rheostat, 

 it was possible to maintain the temperature constant within 

 one or two tenths of a degree. A temperature of 20° was 

 chosen for all the experiments. The disposition of the 

 remainder of the apparatus was the same as in fig. 1, the 

 coil taking the place of the magnet. By choosing a suitable 

 position for the projecting lens OF, it was possible to keep 

 the light reflected from the inner surface of the tube quite 

 clear of the upper and lower extremities of the slit of the 

 spectrometer. 



The insulation of the coil was carefully tested, both 

 between successive layers and between these and the brass 

 tube, under a pressure of 100 volts. No leakage could be 

 detected except in the case of the bottom layer, which was 

 not used in the experiments. 



