342 KENNELLY AND UPSON— HUMMING TELEPHONE. [July 20, 



Tlie amount of cither the primary or secondary extinguishing 

 resistance was found to depend upon the adjustment and operative 

 condition of the transmitter, keeping the receiver, tube-length and 

 all other conditions unaltered. This led to a trial of this method 

 as a practical test of microphone transmitters. 



Observation Scries 5a. Test of Transmitter by Hum-extin- 

 guishing Resistances. — A number of transmitters, some good and 

 others imperfect, were tested under the conditions above outlined. 

 These transmitters were kindly loaned for this purpose by the West- 

 ern Electric Co. Twelve were regular standard instruments that 

 had already satisfactorily passed the factory tests. These were 

 labelled T^ to Tj^ respectively. Four more were marked defective 

 and " down iii volume." They were labelled T^^, T.^-, T^g and Tgi- 

 Four more were marked defective and " thick in quality." These 

 were labelled T,_j, T,-, Tnn and T.,^. Yet another four were marked 

 defective and " burning." These were labelled T^^, J^g, Too ^^'^^ ^21- 

 Defective transmitters " down in volume " are recognized as weak. 

 Those which are of " thick quality " are strong but defective in 

 articulation. Those which are " burning " produce slight arcing, 

 at or near the electrodes, when subjected to normal conditions of 

 operation. 



The results of the tests on these 24 transmitters are given in the 

 accompanying table ; where R represents the primary, and r the 

 secondary, extinguishing resistance, when the transmitter was gently 

 tapped. Care was taken that the observer in this test did not know 

 the label number, or reported condition, of the transmitter under 

 trial. It will be seen that with the good transmitters, the mean 

 primary extinguishing resistances were all included between 26.5 

 and 58.5 ohms, their mean secondary extinguishing resistances be- 

 ing between 1,925 and 4,150 ohms. All of the defective transmitters 

 lay outside these limits, the " down in volume " being low, and the 

 " thick quality " high, in their extinguishing resistances ; except two 

 of the " burning " type, which fell within the good secondary extin- 

 guishing resistance limits. It would seem, therefore, that this re- 

 sistance method constitutes a possible practical application of the 

 humming telephone to transmitter testing ; except that " burning " 

 transmitters may require a separate test for their detection. 



