THE CARBON BUTTON. 15 



vary its resistance by the pressure caused by the vibrations of the dia- 

 phragm. To effect this a small circular piece, technically termed 

 " button," of the semi-conductor was placed between two platinum 

 disks in a small cup. Electric connection between the disks and the 

 button was secured by inserting a small piece of rubber tubing. The 

 first button was made of solid plumbago, and the results were quite 

 excellent ; but still the instrument was inferior to the Bell telephone. 

 Experiments were then made upon many materials in order to obtain 

 a button whose resistance, though small, could be greatly varied ; and, 

 when the list of substances, natural and artificial, had been wellnigh 

 exhausted, without very satisfactory result, a fortunate accident led to 

 the solution of the difficulty. A small quantity of lampblack had 

 been taken from the chimney of a smoking petroleum-lamp and pre- 

 served as a curiosity on account of its intensely black color. This 

 substance was now tried as, it would seem, a dernier ressort. The re- 

 sults were excellent beyond all hope, the articulation very distinct, and 

 the volume several times as great as could be obtained with a magneto- 

 telephone. It was found that the resistance could be varied by pres- 

 sure alone from three hundred ohms to the fractional part of a single 

 ohm. Fig. 3 shows an instrument used for the experimental deter- 



Fio. 3. 



mination of the change of resistance due to pressure only. C is a 

 piece of carbon placed between two metallic plates which are con- 

 nected with the battery, B, in whose circuit is also the galvanom- 

 eter G. As the current passes it must go through the carbon, the 

 pressure on which can be varied by changing the weights placed 

 upon it. The deflections of the galvanometer-needle indicated that 

 the resistance of the carbon varied inversely as the pressure to which 

 it is subjected. The best arrangement proved to be to make the 

 resistance of the circuit ^ of an ohm, while the normal resistance of 

 the carbon itself was three ohms. 



Good results were obtained with other materials besides carbon ; 

 the following is a list of the six most useful substances for this purpose 

 in the order of their value : 1. Lampblack ; 2. Hyperoxide of lead ; 

 3. Iodide of copper ; 4. Graphite ; 5. Gas-carbon ; 6. Platinum-black. 



In the manufacture of the carbon button great care has to be taken 

 that the deposit of lampblack be obtained at the lowest possible tem- 

 perature, and untouched by the flame ; otherwise it is utterly useless 

 for the purpose. Thus commercial lampblack offers very great resist- 



