OF ARTS AND SCIENCES. 87 



two inches in diameter and one fiftieth of an inch thick, one was able 

 to make himself plainly heard, the articulation being unmistakable 

 anywhere in a room sixteen feet square, while persons thirty or forty 

 feet away would know that one was talking : this over a line about half 

 a mile long and ten gravity cells in circuit. With a stick of graphite 

 the results were about the same. In this case the plate touched upon 

 the slightly rounded surface of a stick half an inch square. Of course 

 singing and strong vocal sounds could be heard very much further : 

 they have been heard fifty feet away from the house by persons in 

 passing carriages. The difficulty has been, and yet remains, to main- 

 tain the right pressure of contact. If it be a little too great, the talking 

 sinks to the delicacy of the ordinary telephone ; if it be too little, it 

 breaks up so as to give but little except the pitch of the voice ; and 

 with the micrometer screw it was not possible to do any better than 

 with an ordinary one. If some device can be invented to keep that 

 uniform, the whole problem of the loud-sounding telephone is solved. 

 A mechanical fixture is all that is needed. 



Of all the contrivances tried, the simplest in every way is the fol- 

 lowing sender : An ordinary tin fruit-can, with one end removed, had 

 one terminal from a battery soldered to it ; the other terminal held 

 in the hand so that the circuit was complete when the end of a finger 

 was pressed against the bottom of the can. If, now, one shall talk 

 into the open end of the can, the vibrations of the bottom are suffi- 

 cient to vary the resistance there enough to render it audible in any 

 ordinary telephone. The resistance of the hand is very great, — 2000 

 or 3000 ohms ; this may be lessened somewhat by wetting the hand, 

 and this improves the effect. In like manner, one may fasten the 

 second terminal of a battery to a stick of gas-carbon, two or three 

 inches long, and, holding the tip of it at an acute angle with the bottom 

 of the can, make himself understood by talking into it. 



Most of these investigations have been mechanically carried out by 

 Mr. H. C. Buck, to whose skill and ingenuity I am under many obli- 

 gations ; I have also received considerable assistance of like sort from 

 Mr. W. L. Hooper. To both of them I would express my gratitude 

 for their interest and fidelity. 



It was remarked upon a previous page, that one is limited to but 

 two methods in the making of a speaking telephone. This will be- 

 come evident at once upon the consideration of Ohms's law. All 

 forms of electric telephones depend for their action upon variable 

 electric currents, and hence must conform to the general law of cur- 

 rents. This law is, that the current varies as the electro-motive force 



