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THE POPULAR SCIENCE MONTHLY. 



A Home-made Mierophone. — Some of 

 the readers of this journal may be pleased 

 to have a description of a little microphone 

 that has given good results, and which can 

 be made, in a few minutes, from material 

 at hand. It is represented in the figure of 

 the natural size. It is made from a visiting- 

 card of the ordinary thickness cut square. 

 A round card might look better, but it will 

 give less satisfaction. On the card should 

 be fastened with sealing-wax three thin, 

 light disks of carbon, BBB', of the qual- 

 ity used in the electric light. The disks 



ters, for example, by the terminal D', follows 

 the rod C, then the disk B', whence by the 

 wire b it passes by the two disks B to return 

 to the terminal D through the two rods C C. 

 The little instrument may be made very 

 sensitive to the voice and to all sounds, pro- 

 vided the card A is given the proper weight, 

 and is neither too heavy nor too light. The 

 voice, with its timbre, of a person speaking 

 in his usual tone at the other side of the 

 room, can be heard very distinctly in it. 

 The sounds of the piano are particularly well 

 reflected. The apparatus should be placed 



should be placed symmetrically at the an- 

 gles of an equilateral triangle, and should 

 be put in communication with each other by 

 the copper wires h h h\ which are either sol- 

 dered or stuck tightly into holes made in 

 each disk to receive them. Platinum may 

 be advantageously substituted for copper. 

 The rest of the apparatus consists of a 

 square wooden foot, M, supporting three 

 prismatic rods of carbon, C C C ', arranged 

 so as to correspond exactly with the three 

 disks BBB'. Two of the rods, C C, com- 

 municate by the copper or platinum wires 

 dd with the common terminal D, while the 

 third rod, C, communicates alone with a 

 second terminal, D'. The upper ends of the 

 charcoal rods should be cut into a bevel- 

 shape — not into a point, for that does not 

 give sufficient contacts. The rods are sealed 

 to the wooden base M. The theory of this 

 microphone is very simple. The current en- 



upon a table two or three metres away from 

 the sound. For a battery to put the micro- 

 phone in action, I have generally used a small 

 Bunsen element. Two or three Leclanch6 

 elements would do as well. I have used a 

 modification of the Leclanch6 elements, in 

 the shape of a pile made of a plate of zinc 

 and a carbon plate, moistened with a satu- 

 rated solution of bichromate of potash and 

 hydrochlorate or sulphate of ammonia. It 

 is in fact the bichromate pile without the 

 costly mechanism which is used for reliev- 

 ing the zinc from the action of the acid 

 when the apparatus is at rest. This ele- 

 ment does not waste when the current is 

 interrupted, as in the Leclanche pile. A 

 difficulty which arises in the use of the pile, 

 from the penetration of the carbons by the 

 ammoniacal solutions till they attack the 

 wires, has been obviated by M. Preaubert's 

 device of exposing the carbons to a bath of 



