16 THE POPULAR SCIENCE MONTHLY. 



ance to the passage of the electric current, and for that reason can not 

 be used at all. The lampblack taken from the chimney is laid upon a 

 white slab, where the brown portions are readily detected and removed. 

 The pure black portion is then ground and subjected to a pressure of 

 several thousand pounds in a mold. It is then repowdered and re- 

 pressed several times, and finally molded into buttons weighing three 

 hundred milligrammes each. 



The special advantages of the carbon button over buttons of other 

 materials are notably its sensitiveness to very slight changes of pres- 

 sure, its remarkable elasticity and its delicacy over a long range of 

 absolute pressures. These properties it possesses in a higher degree 

 than any other substance, and the explanation of this peculiarity has 

 been found in certain of its physical characteristics. Microscopic ex- 

 amination has shown that, of all finely divided substances, whether 

 obtained by chemical or mechanical means, lampblack is the most 

 finely divided. Now, it is known that the change in resistance of any 

 piece of finely divided material, caused by change of pressure, is due 

 to the increase or diminution of the number of particles brought into 

 contact with each other. On this account a given change of pressure 

 will show a greater change of resistance in carbon than in any other 

 substance. Moreover, with other materials, a point is soon reached 

 when additional pressure ceases to produce any appreciable change in 

 resistance, doubtless because all the particles are already in contact. 

 But the fact that lampblack is so finely divided enables it to respond 

 to changes of pressure long after other materials have lost their sensi- 

 tiveness. For this reason a comparatively large initial pressure can be 

 used with the carbon, and the instrument is not so easily thrown out 

 of adjustment. That the greater delicacy of the lampblack is due to 

 the fact that it is so finely divided has been confirmed by experiments 

 made with gas-retort carbon, the particles of which are comparatively 

 coarse, graphite, which is more finely divided, and lampblack, whose 

 particles are the finest of all. The changes of resistance for a given 

 change of pressure were found to be proportional to the number of 

 particles in a given volume, or inversely proportional to the size of the 

 particles. By microscopic comparison between a Rutherford diffrac- 

 tion grating having 17,291 lines ruled to the inch on a piece of specu- 

 lum metal, Mr. Edison estimated that there could not be less than 

 10,000,000 points in contact in the carbon-button when used in the 

 telephone. This must, however, be regarded only as an approxima- 

 tion. 



The only defect in the carbon button is its friability. But, when 

 properly armatured, it need receive no violent shock, and will last as 

 long as necessary. Even if it should happen to become cracked, the 

 volume of sound would not be materially lessened. Experiments have 

 been made to harden the button by mixing various substances with 

 the carbon, and then subjecting the mixtures to high temperatures. 



