664 Prof. J. Patterson on tlie Electrical 



resistance of both measured. These films are marked with 

 the suffixes A and B. The greatest difference between the 

 resistance of two films deposited at the same time occurs in 

 35 A and 35 B, where the difference amounts to 11 per cent. 

 The least difference occurs in 34 A and 34 B, and amounts to 

 2 per cent. The films are, moreover, very thin, so that the 

 agreement is fairly close. 



If the specific resistance of the films is constant and the 

 rate of deposition constant, then the resistance per cm. square 

 multiplied by the time of deposit is a constant, and the thick- 

 ness is directly proportional to the time of deposit. As will 

 be seen from the last column in Table V. the product of the 

 resistance per cm. square and the time of deposit varies very 

 much for the thicker films. In the case of the thinner films 

 or those deposited in a minute or less the product is too great 

 to be accounted for by variations in the conditions, but it 

 would seem to be due to a change in the specific resistance. 

 There was no attempt to keep the conditions constant when 

 depositing the film, so that the variation of the product of 

 resistance multiplied by time of deposit for the thicker films 

 was probably due to changes in the induction-coil, and, 

 moreover, the films were not deposited in succession, so that 

 this would also make a difference. 



To obtain a rough approximation of the thickness of the 

 films, which were too thin to be measured by Wiener's 

 method, the average value of the rate of deposit per minute 

 of the films whose thickness could be measured was taken, 

 and the thickness of the others calculated from that and the 

 time of deposit. The values of the thickness of all the films 

 not marked with an asterisk were calculated oil the above 

 assumption. 



The above results would indicate that the specific resistance 

 is fairly constant for all thicknesses above 10 -6 cm., and 

 below this value the specific resistance changes very rapidly. 

 Another set of platinum films was made, and this time the 

 conditions were kept as constant as possible. Anew cathode 

 6 cm. in diameter of rolled platinum 1/10 mm. thick was 

 used. The induction-coil, which gave a 6-inch spark, was 

 adjusted to run very steadily. The films were deposited in 

 succession, and the depositing apparatus was never taken off 

 the pump for a longer period than was necessary to change 

 the glass strips. It usually required about one hour to make 

 a film. After the film was removed from the bell-jar in 

 which it was made, it was put into a glass tube and protected 

 from the air and dust. Four films thick enough to be 



