i 4 THE POPULAR SCIENCE MONTHLY. 



account of the decomposition of the fluids. These were therefore 

 abandoned and the attempt was made to vary the strength of the cur- 

 rent by the use of platinum points, springs, and other devices. The 

 number of these points which was to be brought into the electric cir- 

 cuit was to be dependent upon the amplitude of the vibration, and 

 thus the resistance of the circuit was to be varied inversely as the 

 intensity of the sound producing the vibration. All of these con- 

 trivances were of no avail. Subsequently plumbago and white Arkan- 

 sas oil-stone were tried on account of their great resistance, and with 

 these fair success was attained. Various expedients were used to 

 make the portion of the material employed in the circuit proportional 

 to the amplitude of vibration, but the confusion introduced by the de- 

 vices themselves rendered the apparatus practically useless. All these 

 experiments were conducted before the close of the year 1876. 



In January of the next year the idea occurred to Mr. Edison to 

 make use of the fact that semi-conductors vary their resistance with 

 the pressure to which they are subjected a thing which he had acci- 

 dentally discovered while constructing some apparatus for artificial 

 cables about four years before. He immediately set to work to con- 

 struct an instrument. A diaphragm carrying at its center a spring 

 faced with platinum was placed opposite to a small cup containing the 

 semi-conductor to be tried. The adjustment was secured by means of 

 a screw fastened to the cup. The vibrations of the diaphragm pro- 

 duced by the tones of the voice determined the pressure of the spring 

 upon the semi-conductor. The materials first experimented upon were 

 crude plumbago mixed with dry powders of different kinds. The 

 results obtained were encouraging, the volume of sound being great, 

 but the articulation so poor that s%ne practice was necessary before 

 the peculiar sound of the instrument could be caught with ease. An 

 improvement was effected when, after much experimenting, solid mate- 

 rials were abandoned and tufts of gloss silk coated with semi-conduc- 

 tors were substituted. But, with all the improvement that could be 

 devised, the instrument was still very inferior to the magneto-telephone 

 of Professor Bell, and required such frequent adjustment as to make 

 it very objectionable. Experiment developed the fact that the change 

 in resistance in the semi-conductor, due to the impact of sound- vibra- 

 tions, was very small, and, in order to make this change of resistance as 

 important a factor as possible, Mr. Edison determined to make the 

 resistance of his circuit very small : to that end he tried the primary 

 circuit of an induction-coil, but the experiment failed. The cause of 

 failure was at first only a matter of conjecture ; but, by trying one 

 thing after another as they suggested themselves, without any very 

 definite purpose, conjecture finally condensed into the belief that the 

 resistance of the semi-conductor was too great to be used with the 

 primary circuit of an induction-coil. The effort then was to reduce 

 the resistance of the semi-conductor to a few ohms and still be able to 



