668 Prof. J. Patterson on the Electrical 



Longden* found that films having a resistance of 53 ohms 

 per cm. square had a temperature-coefficient of —0*0003. 

 This is somewhat smaller than those obtained for films of 

 about the same resistance. In the above table all the co- 

 efficients are positive and too large to be negligible. Longden 

 found, however, that for resistances from about 670 to 14000 

 ohms per cm. square the temperature-coefficients were negli- 

 gible or zero, and above this they were negative. Moreover, 

 he found that the artificial ageing of the films depended very 

 much on the temperature-coefficient, and that if the tempe- 

 rature-coefficient was negligible there was very little if any 

 artificial ageing required. From Table V. it will be seen 

 that in the experiments described in this paper, there was a 

 very great decrease in the resistance of the films on heating, 

 and that the greatest decrease occurred in those films which 

 came within the limits of Longden's results, for no tempera- 

 ture-coefficient and no ageing. Longden used the Wehnelt 

 interrupter on his induction-coil, and it may be that the 

 rapidity with which the films are deposited has a very marked 

 effect on the results. It is more probable, however, that the 

 different results are due to different degrees of purity of the 

 cathode. It is well known that a very small amount of 

 impurity has a very marked effect on the resistance of plati- 

 num and the temperature-coefficient. Owing to the exceed- 

 ingly small quantity of metal in the films a very small amount 

 of impurity in it could cause all the difference. Besides, the 

 rate at which the impurity is disintegrated from the cathode 

 may be very much greater than for the platinum itself 

 (Recent Researches), and consequently the amount of im- 

 purity in the film would be greater than that in the cathode. 



Resistance of Silver Films, 



The silver from which the films were deposited w^as not 

 chemically pure although the amount of impurity in it was 

 very small. 



Silver deposits very rapidly in the cathode discharge so 

 that it was very difficult to get a film. The films were often 

 crystalline in appearance or powdery so that they rubbed off 

 the glass ; this was especially true of the thicker films. By 

 letting the induction-coil run very lightly so that the deposit 

 was slow very good films were obtained. The depositing 

 apparatus was exhausted until the dark space was about half 

 way between the cathode and the glass strip. This degree 

 of exhaustion was found to be better for the silver films than 

 exhausting until the dark space just reached the glass strip. 

 * Physical Review, vol. xi. p. 84 (1900). 



