PAPERS ON CHEMISTRY AND PHYSICS 305 



treatment, the exact thickness of the film and the exact 

 condition of the contacts, etc. 



The following' characteristics of carbon films as high 

 resistances have been noted : 



I. Changes in resistance produced by processes of 

 artificial aging are followed usually by a recovery in 

 which the resistance tends, partially at least, to reas- 

 snme its former magnitude. This phenomenon is illus- 

 trated by the curve in Fig. 3. Here a film has been sub- 

 mitted to alcohol vapor and heat treatments. 



II. Bathing a carbon film in alcohol vapor produces 

 a decrease in resistance except in the following cases : 



(a.) When one or both of the contacts between the 

 carbon and silver are poor, the carbon is washed away 

 from the silver producing an increase in resistance. 



(b.) If the alcohol vapor is forced on to the film in 

 such a manner as to disturb the mechanical arrange- 

 ment of the particles of carbon, an increase in resistance 

 will be noted. 



(c.) If the alcohol vapor is applied repeatedly, with- 

 out intervals of natural recovery between the applica- 

 tions, a limit is reached, after which no decrease can be 

 produced. If the process is carried still farther, the re- 

 sult is an increase similar to that produced by natural 

 recovery, but more rapid. It has been impossible to ex- 

 plain why this occurs, but trials in which the contacts 

 were covered completely with paraffin have proved that 

 these seemingly erratic variations are not due to defects 

 in construction, and may be repeated quite regularly. 

 This effect is shown also in Fig. 3. 



III. Heating a film at a temperature from 80° to 

 110° C. produces a decrease in resistance due probably to 

 the mechanical change in the arrangement of the parti- 

 cles of carbon when the glass and carbon expand and con- 

 tract. (See Fig. 3.) Heating to too high a temperature 

 usually causes some cracking of the silver copper tips, 

 which will result in an increase in resistance. 



IV. Bathing a film in liquid alcohol decreases the re- 

 sistance except in those special cases described under 

 the alcohol vapor treatment. As with alcohol vapor, an 

 extended treatment in the liquid alcohol produces an in- 



