656 Prof. J. Patterson on the Etectrical 



Method of Measuring the Thickness of the Film. 



To determine the specific resistance o£ the films it was 

 necessary to measure both their resistance and thickness as 

 accurately as possible. As the films were too thin for their 

 thickness to be determined by weighing, Wiener's * inter- 

 ference method was used. In determining the thickness by 

 this method, the best optical glass had to be used to obtain 

 any accuracy, and, moreover, in measuring the thickness the 

 films had to be spoiled for measurements of resistance, be- 

 sides, putting electrodes on the optical glass would spoil it, 

 hence it was necessary to make two films at the same time, 

 one on the optical glass C (fig. 2) for measuring the thick- 

 ness, and the other on the glass strip D (fig. 2) for the 

 resistance. 



Since the cathode was at least 6 cm. in diameter, and as 

 the glass strip and optical glass when placed side by side 

 formed a square 25 mm. to a side, and were placed as nearly 

 central as possible, it was assumed that the deposit was 

 uniform over these surfaces. This was tested by depositing 

 films on two pieces of glass and measuring their resistance. 

 The results agreed closely enough to warrant the assumption. 



The optical glass was about 25 mm. long and 12*5 mm. 

 wide. A strip of very thin mica about 4 mm. wide was 

 placed over the optical g ] ass, as shown in fig. 3. This pre- 

 vented the film from depositing on that portion of the glass. 



Fig. 3. 



>» Mica strip 





When the films w r ere deposited they were taken out of the 

 bell-jar, and the optical glass was then replaced without the 

 mica strip and another film deposited on it. By this means 

 a metallic surface was obtained over the whole surface of the 

 glass (fig. 4), and the part A was thicker than the part B by 

 the thickness of the first film. The metallic surface also 

 ensured that the change of phase of the reflected light would 

 be the same at both parts A and B. A piece of optically 

 plane parallel glass was placed over the metallic surface and 



;* Wied. Ann. xxxi. p. 629. 



