The Invention of the Telephone

Exhibit 5: How to Build Modern Versions of Early Speaking Telegraphs

Materials and Links contained in this document originally came from "Person to Person" Exhibit of the National Museum of History and Technology Smithsonian Institution. December 1976

Liquid Transmitter

Click to view the liquid transmitter
This was the device over which Bell spoke to Watson on March 10, 1876: "Watson come here, I want to see you." The principle is the same for the carbon transmitter, but contact is made between metal and water.Basic elements are a couple of plastic or paper cups of different shapes, a nail, and some wire. Or use your own imagination.
One cup should fit in the other as shown, so that a nail (or screw, or needle...) poking through the top cup can touch the surface of the water in the bottom cup. One end of a wire (perhaps attached to a washer to hold it down and provide better electrical contact) is in the water. The end of another wire is attached to the nail. And you have your transmitter.
It operated the same as the carbon transmitter. Some additional care must be taken however. The nail should be dry except where it touches the water, and the amount in contact should be small. And you probably won't want to use more that 3 volts, because otherwise too much gas is given off at the contact and only its noise can be heard. Best results are obtained if the positive pole of the battery is attached to the nail, since the oxygen is given off at the contact point; hooked up the other way, twice as much hydrogen would be given off, producing twice as much noise.

Magneto Receiver or Transmitter

Click to view the magneto transmitter/reciever
Bell's first practical telephone had identical transmitter and receivers. They both operated on a electro-magnetic induction principle (as does the receiver in your telephone today): a changing magnetic field produces a fluctuating current, and a fluctuating current produces a changing magnetic field.
To make an instrument of this type you need a tin can (not aluminum since you need iron on it), wire, and iron bolt, and a piece of wood for support. Wind the bolt with a few turns--perhaps 20-30--of wire and attach the ends to the battery; this gives you an electromagnet. Wind a similar number of turns around the bolt with a second piece of wire; its ends are the terminals of your instrument. Mount the bolt vertically on a piece of wood so that its top end is just under (by about 1/16 inch) the end of the tin can.
You are now finished. Speaking against the end of the can (the diaphragm) moves it near the electromagnet, inducing a current in the coil windings. Or, as a receiver, the incoming signal causes the magnetism of the bolt to change, moving the end of the can back and forth. Incidentally, when used as a receiver, the battery doesn't need to be hooked up; you don't even need it for the transmitter if the bolt is replaced by a permanent magnet. You can improve the arrangement by making a cone above the diaphragm, as shown.
In the second drawing is another form of this instrument. Wrap a cardboard tube (as from a toilet tissue roll) with about 100 turns of wire. Hang a small magnet down into it from a diaphragm. You then have a transmitter or receiver that operates in a fashion similar to the one above.
In making any of these instruments you may want to make your own variations. In doing so, you will only be following the sort of experimental approach pursued by the early telephone inventors one hundred years ago. Have fun, and good luck.

Carbon Transmitter

Click to view the carbon transmitter
The principle involved is to have the vibrations of the voice produce changes in an electrical resistance. This may be done by having it affect the contact between two conductors. Many materials can be used. Bell and Gray used metal in contact with water; Reis used metal against metal; but the most effective results were obtained using carbon, and various forms of carbon transmitters were developed by Blake, Berliner, Hughes, Ader and Edison. And this is the type of transmitter still used in most telephones today. In all of these devices two or more pieces of carbon (sometimes metal and carbon) are in loose contact; pressure of the voice on a diaphragm presses the pieces closer together, reducing the resistance to the flow of electrical current between them. The result is an electrical current that varies with the fluctuations of the voice.
Our microphone is similar to that which David Hughes invented in 1873. All that are needed are a couple of pieces of some form of carbon (like pencil leads or the center rods from flashlight batteries), wire, (almost anything will do, but a small gauge is easier to use) and stiff cardboard or thin wood to act as a diaphragm. Many arrangements can be used and you may want to if you can design a good one. We show two in the pictures here. In one, a pencil, sharpened at both ends to expose the lead, rests loosely between two battery rods mounted on the diaphragm. In the other, one or more battery rods rest across two others which are fastened to the diaphragm. Wires are attached to the carbon pieces on the diaphragms, and you have a microphone.
To make it work, include a battery (3 to 6 volts should do fine) and run your wire to the receiver. With some care, a little luck, and a quiet area, you ought to be able to make it work with your own magneto receiver. If you have a loudspeaker or earplug from a small transistor radio, these should work well. Or you can use a tape recorder. In this case, you must keep the battery's direct current out of the recorder. Using a transformer will transfer the fluctuating current produced by the voice, but not the direct current. You can purchase a small transformer from an electronics store or make your own. To do so, simply wind 10 or 20 turns of a small-gauge insulated wire (enamel coated wire is available in many hardware stores) around an iron bolt or a bundle of nails, leaving the two ends free. On top of this wind another piece of wire about 100 turns. The ends of the first wire are attached to your transmitter, the ends of the second go to the microphone input of your tape recorder. And now you are ready to record.

Photophone Transmitter

Click to view the photophone
In 1881 Bell transmitted voice signals over a light ray. His diaphragm was a mirror, and the direction at the spread of the light reflected from it varied slightly with the vibrations. The receiver was a piece of selenium, which has an electrical resistance sensitive to light. The resistance of the selenium therefore varied with the voice fluctuations, producing variations in the current through it, which could be heard on an ordinary receiver.
You can do as Bell did. Or, somewhat easier, do the similar experiment shown here. You need a silicon solar cell (available inexpensively at an electronics store), a tin can, some wire and a flash light. Arrange the pieces as shown. Get a good polish on the bottom of the can; you may even want to attach a separate piece of mirror or polished metal to it.
There is a difference here from Bell's experiment. Since your silicon cell is producing it's own current, no battery is needed. This means it is also producing direct current from the steady part of the beam of light, so include your transformer if you use the recorder.
Interesting links on telegraphs

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This page was last edited: Sunday, July 18, 1999