12 



KEXNELLY AND KUROKAWA. 



Techniqiw of Mcasiirrmcnis: A series of measurements is first made 

 of the apparent resistance R and inductance L of the recei^'er z, 

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

 clude the resonant frequency f^ of the receiA'er, which is allowed to 

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

 ments of R and L at Aaried frequency, but constant testing-current 

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

 or prevented from ^-il)rating. This damping may be effected in 

 several ways, that have been described in preceding pul)lications.^ 

 A convenient procedure, howcA'er, has been found to consist in form- 

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

 wire, so as to press gently, l)ut adherentl}-, on the center of the dia- 

 phragm, and also across a diameter of the receiver cap, as is indicated 

 in Figure 5. liy this means, the diaphragm is prevented from vibrat- 



FiGURE 5. Diagniin iuclic:itiiis the mode of clumping a Receiver Diaphragm 

 by the application of a Wax Bridge or T. 



ing without altering the normal airgap. When great precision is not 

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

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

 hole in the recei^'er cap, so as to plug this hole without touching the 

 diaphragm. This method of damping the receiver has been called 

 "acoustic damping." It permits of some vibration of the dia])hragm, 

 especially towards higher impressed freciuencies ; but the cushioning 

 of the air, by the inserted plug, prevents the vibration from exceefling 

 a small fraction of that which occurs with the plug withdrawn.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



7 Bibliography 7, 9. 



