12 



KENNELLY AND KUROKAWA. 



Technique of Mcasureiiienis: A series of measurements is first inafle 

 of tlie apparent resistance R and inductance L of the receiver z, 

 Figure 4, over a fairly considerable range of frequency; so as to in- 

 clude the resonant freciuency f^ of the receiAcr, which is allowed to 

 \abrate in the free air of the testing room. This series of measure- 

 ments of R and L at A'aried frequency, but constant testing-current 

 strength, is then repeated without delay, with the diaphragm damped, 

 or prevented from A-ibrating. This damping may be effected in 

 several ways, that ha^'e ])een described in preceding publications.^ 

 A convenient procedure, however, has been found to consist in form- 

 ing a T of paraffin wax, and melting the extremities of this with a hot 

 wire, so as to press gently, l:)ut adherently, on the center of tlie dia- 

 phragm, and also across a diameter of the recei\'er cap, as is indicated 

 in Figure 5. I^y this means, the diaphragm is preA'ented from vibrat- 



I 



Figure .". Diagram imlioat ing the mode of clam])ino; a Receivei- Diaphragm 

 by the apphcation of a Wax Bridge or T. 



ing without altering the normal airga}). AMien great precision is not 

 desired, a still swifter and simpler method of damping the recei^'er, is 

 to insert a round plug of wood, wax, or other solid material, into the 

 hole in the receiver cap, so as to plug this hole without touching the 

 diaphragm. This method of damping the recei\'er has been called 

 "acoustic damping." It permits of some vibration of the diaphragm, 

 especially towards higher impressed f retiuencies ; but the cushioning 

 of the air, by the inserted plug, pre^'ents the A'ibration from exceeding 

 a small fraction of that which occurs with the plug withdrawn. In 

 other words, the acoustic impedance Z2 of such a plug and air cushion 

 is A'ery large. 



7 BiliHography 7, 9. 



