48 



BELL SYSTEM TECHNICAL JOURNAL 



tor>'. If \vc remove the air and measure tlie resistance at (iiftercnt 

 voltages, we may expect to find ciianges in the resistance similar 

 to those indicated in F"ig. 2. Curves 1 and 2 were talcen for increasing 

 voltages. The return curves were taken with decreasing voltage. 

 Removal of the air from this particular sample of carbon produces 

 comparatively large changes in the resistance. The resistance at 1 J 2 

 volts is se\eral times that at 48 v'olts. These curves were taken 

 under conditions wherein all of the other factors were controlled. A 

 sufficient number of obscr\ations was made in each case in order to 

 establish the probable errors of the points as indicated b\- the radii of 



Fig. 3 — rVissililc Types of Breathing of Gratuilar Carlinti Microphone. 



the circles. If this same experiment were carried on at a difTcrent 

 temperature, radically different results would be obtained. 



If, instead of allowing the current to flow for a short inter\al of 

 time, a continuous record is made of the resistance of the carbon 

 while practically constant current Hows through the carbon, the 

 resistance will be found lo \.ir\ . Tlu- niaxinuini resistance n-aiiu-d 

 in certain instances nia\- amount lo M'veral limes the minimum \aliic. 

 In general, this phenomenon is attributed to the effects of gas .sorbcd 

 on the surface of the material. Transmitters cannot be made of 

 lavite so that the exixuisions and contractions of the piece parts 

 thereof augment the changes in resistance. This phenomenon, 

 termed "breathing," may be, but seldom is, regular or periodic. .\n 

 exceptional case of bri'athing is shown in Fig. S. This was obi. lined 

 with a special ty|)i' nl cailidii in .i ((ininu'ici,!! ^tniriurc. The rur\t'> 



