Alexander Graham Bell developed the basic ideas of the telephone while he worked in a multiple telegraph. A demonstration during the Exhibition of the Centenary in Philadelphia (Pennsylvania), in 1876, sent its invention to everybody; a year later founded the Company of Bell Telephones. The basic set of the invention of Bell was formed by an emitter, a receiver and an only cable of connection. The emitter and the receiver were identical and contained a flexible metallic diaphragm and a magnet with form of horseshoe within a coil. The sound waves that affected the diaphragm made it vibrate within the field of the magnet. This vibration induced an electrical current in the coil, that varied according to the vibrations of the diaphragm. The current traveled by the cable until the receiver, where it generated fluctuations of the magnetic field strenght of this one, causing that its diaphragm vibrated and reproduced the original sound.

The telephone that we will construct is a reproduction of one of the first coal telephones: the telephone of coal bar of Hughes. It works with an earpiece whose construction is also quite simple. Our telephone, like the one of the mentioned scientist, has as it divides main one trumpets of mineral coal.

We will use a mine of a black pencil “hard” type.


We will use the following materials: a small box of cardboard or wood, a black pencil type HR, disposable razor blades, screws, wire of conección, batteries, wire of enameled copper No.16, iron bar or several great nails and a wood block.


First we must construct the carbon microphone, parabesto become some you cut parallels with a blade to the center of the small box that will be our base. Each cut must be to about 5 cm of distance. The razor blades are taken and it introduces them in raburas that became to the small box so that they are assured in that position as it is seen in figure 2. Soon some are enrrollan thin wires to each one of the razor blades later to make the conecciones. Now the estremos of a black pencil of the hard type are peeled (mineral coalmine) and is placed it in the edges of the razor blades and not in the wood of the pencil. He is better to completely clear the mine of the pencil as it is seen in figure 2. Now we come to the construction of the earpiece; this one takes control of a metallic cover of spiral, that will work as the diaphragm of the earpiece. It must be of brass or a similar metal, that is to say, that can be attracted by the electromagnet. Great and at the most thin the cover will work far better. The cover, that we will call diaphragm from now on, maintains by the edge with three tacks or screws of wide head without tightening too much. It must mount in a wood piece in such a way that it is attracted by the electromagnet, but without it touches it. The electromagnet consists of one trumpets of iron (nails in their place can be used), on which several hundreds of returns of enameled or finer Not 16 copper wire become involved. The more thin the wire and whatever the more returned have, better the operation of the earpiece. The electromagnet is placed in center in a perforated hole of the wood. Finally the earpiece is taken and the conecciones become as it is in figure 1. In order to prove the conecciones the receiver (auricular) near the ear is placed and soon we move the pencil with the fingers; we must listen to a noise in the earpiece. Now we can use the telephone to transmit the voice, which takes control of a companion whom it will indicate to us if it can listen to claramante while we spoke in direction of the pencil.


VIDEO: How to make a Telephone of Bell

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