336 



Profs. Remold and Rticker on the 



[June 21, 



foreign metal which might give rise to local galvanic action when the 

 current passed through the film. At the bottom of the glass vessel was 

 a small porcelain dish containing mercury, in which was placed a platinum 

 crucible, the lower part of which was amalgamated to ensure good con- 

 tact with the mercury. The diameter of the mouth of the crucible was 

 as nearly as possible the same as that of the platinum cylinder above 

 mentioned. To obviate the possibility of the film thinning by eva- 

 poration from its surface, a little of the liquid used was placed in the 

 bottom of the glass vessel, and the platinum crucible was also filled with 

 the liquid to within about 1 millim. of its upper edge. The cylindrical 

 films were produced in the following manner. A plane film was formed 

 on the platinum cylinder, which, when the cover was replaced on the 

 vessel, was blown out into a bubble through the india-rubber tubing. This 

 bubble, when large enough, adhered to the edge of the platinum crucible, 

 and both the quantity of air within it and the position of the crucible 

 were then so regulated as to make it as accurately cylindrical as possible. 

 The edge of the platinum cylinder was levelled by altering the position 

 of the whole apparatus until all points on its edge were, as was 

 determined by the help of a cathetometer, in the same horizontal plane. 

 With this arrangement it was easy to measure the resistance of the 

 ' film. A binding-screw on the smaller portion of the ebonite cover 

 was connected by a platinum wire with the mercury on which the 

 platinum crucible rested, while another binding-screw on the brass tube 

 formed the point of connexion with the upper electrode. The resistance 

 was measured by means of a Wheatstone's Bridge (of the Post-Office 

 pattern), and as it continually changed, and nearly always slowly but 

 steadily increased, the known resistance was made up to a certain 

 amount, and the moment when the unknown resistance reached that 

 amount noted. 



As the resistances to be measured were very large, a box of resistance- 

 coils was introduced into one arm of the bridge containing ten resistance- 

 coils of about 100,000 ohms each. The actual resistance available for 

 purposes of measurement was thus rather greater than 1,000,000 ohms, 

 and by the multiplying power of the instrument resistances up to 100 

 times this amount could be measured. The galvanometer used was a 

 reflecting instrument of 5000 ohms resistance. In the arm of the balance 

 which contained the film-resistance a commutator was placed, and every 

 time the key was depressed the direction of the current was changed. By 

 this means error due to polarization was reduced to a minimum. The 

 battery consisted of three Grove's cells. 



The electrical observations were made in a room adjoining that in 

 which the film under experiment was placed, and at the same time a 

 second observer measured with a cathetometer the breadths of the bands 

 of colour exhibited by the film. This operation was repeated at least 

 twice, and the time of each observation was noted, so that the rate of 



