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Page:Popular Science Monthly Volume 90.djvu/655

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Popular Science Monthly

��639

��This method can be used nicely for finding the capacity of an antenna if the ground and aerial leads are placed across inductance C in just the same manner as when connecting a condenser to be measured.

If it is found that no zero-signal point can be found by varying G, try reversing the connections of one of the coils {D, for example). If still no zero-point is found, the condenser G is probably of different value (throughout its range) than con- denser H. To get silence in the tele- phones when the condensers are equal, the coupling oi A to B should be the same as that oi C to D; otherwise there will be only a reduction of sound at the balancing point.

��How to Make an Experimental Dictograph

THE word dictograph is a trade name used to designate one make of telephonic instrument for re- ceiving, transmitting and magnifying sounds. In a compact form it may be concealed and used in various kinds of detective work, conveying to the waiting listener, unknown to the suspected person, what is being said.

Such an instrument can be easily made by any one having even a small amount of knowledge of electricity. All instruments of this class are a com- bination of a telephone receiver with some form of a microphone, which is used as a transmitter. Such an instru- ment consists of a cell of granular carbon between two carbon electrodes. The back electrode is fixed, while the front electrode is carried on a thin iron diaphragm. The carbon disks and the granular carbon held between them constitute a powerful microphone. On account of the weight of the iron dia- phragm and the electrode attached to it the inertia of these parts is compar- atively great so that it is not affected by ver>' faint vibrations, and to transmit the voice properly the person using it must be close to the transmitter. To make a microphone useful for the dicto- graph it must be so sensitive that it will reproduce the voice when the sf)eaker is a distance away. This may be accom- plished by using a microphone in which the only moving part is a light carbon diaphragm. An instrument of this type

��is shown in detail in the illustration.

A carbon button containing on one face a number of depressions is held near a carbon diaphragm. The button forms one electrode and the diaphragm the other. The current flows from the button to the diaphragm through th^ granular carbon which half fills each depression in the button. The atmos- pheric vibration set up by the voice causes motion of the carbon diaphragm which is transmitted to the granular carbon, increasing or decreasing the pressure of the carbon grains against each other, and causing variations in the current that will reproduce in the receiver the sound of the voice which caused the vibrations.

A transmitter of this general t>'pe, together with a telephone receiver, a switch, and two battery cells, con- nected as shown, are the essential parts of all instruments of the dictograph type, as well as for all telephonic aids to the deaf, any one of which may be used as a dictograph by employing leads of proper length to allow the instrument to be placed as desired.

The experimenter can find the parts necessary for making the transmitter listed in catalogues of electrical supply houses — carbon backs, carbon dia- phragms and grain carbon. In making the transmitter care must be taken to place the diaphragm so that it does not touch the carbon back and still is close enough to prevent carbon grains from

���Connections for the battery circuit and a cross section of the transmitter

getting between the diaphragm and the carbon back.

The transmitter must be held in a vertical position, or the grain carbon will not He against both the diaphragm and the carbon back. A switch is provided for opening the circuit.

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