308 ILLINOIS STATE ACADEMY OF SCIENCE 



crease. It is evident from examination of a film so treat- 

 ed that an extended liquid alcohol treatment washes off 

 all loose particles of carbon and an increase in magni- 

 tude necessarily follows. The ordinary decrease noted 

 at first with either treatment is due to the packing and 

 hardening effect of the alcohol on the film. However, in 

 depositing a film from a camphor flame, it is very prob- 

 able that the particles of carbon in leaving the flame carry 

 with them camphor vapor which is incompletely burned, 

 and which becomes occluded in the film. It is possible 

 that the natural change in resistance of an untreated 

 film is due to physical or chemical changes in this occlud- 

 ed material. This camphor is washed to the surface of 

 the film by the alcohol vapor treatment, and is in a large 

 part removed completely by the liquid alcohol bath. For 

 this reason, the liquid bath is preferable to the vapor 

 treatment. 



With this knowledge of the effects of artificial aging 

 on the resistance of thin carbon films, it is possible to 

 prescribe the following treatment in the preparation of 

 such films for use : 



It is desirable to cleanse the film as far as possible 

 from all foreign matter and loose particles of carbon, 

 thus bringing the resistance of the film as nearly as pos- 

 sible to its permanent magnitude. This is done most ef- 

 ficiently and quickly by immersing the film in liquid alco- 

 hol for ten to twenty minutes, and then drying for a half 

 hour at 60° to 70° C. in an oven. The change in resistance 

 of a film prepared in this way is shown in Fig. 4. 



Fig. 5 shows by comparison the difference between an 

 untreated film under the influence of natural aging alone, 

 and a film which has received the above prescribed treat- 

 ment. Here the per cent of change in resistance per hour 

 is plotted against the age of the film. It will be seen 

 that the change in magnitude of the treated film becomes 

 much less at an early age than that of one which has 

 received no treatment. The finished film should be 

 mounted as described above, for protection and conven- 

 ience in use. For best results the films should not be 

 covered with paraffin or shellac since these materials 

 cause undesirable changes in resistance with changes of 



