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. 



