PIERCE. — A METHOD OF MEASURING THE INTENSITY OF SOUND. 381 



3. The resistance of the secondary of the transformer and that of the 

 galvanometer should be high because the resistance of the crystal for a 

 small current is several thousand ohms. 



Guided by these considerations, and by the results of preliminary 

 experiments with several small induction coils, two transformers were 

 wound, of which the one that proved the more satisfactory had the 

 following dimensions : 



Length of iron core, 9.5 cm. 

 Diameter of iron core, 1 cm. 

 Depth of channel, 1.5 cm. 

 In this channel were three coils of which either pair could be used as 

 primary and secondary. These three coils had respectively 16, 280, 

 and 720 ohms resistance. 



With this transformer experiments were made with three different 

 telephones, of which a Siemens and Halske " Lautsprecher," rewound to 

 466 ohms, and provided with a small conical sound collector 10 cm. in 

 diameter, proved the most sensitive. This telephone was ordinarily 

 used with the 280 ohm primary and the 720 ohm secondary. The 

 other two telephones used had resistances of 53.8 and 99.8 ohms re- 

 spectively, and were used with the 16 ohm primary and the 720 ohm 

 secondary. 



Experiment I. Adjustment of the Receiving Telephone Circuit to 

 Resonance with the Sound. — After having made a preliminary selec- 

 tion of the pitch to be employed in a particular experiment, it becomes 

 important to adjust the electrical circuit to resonance with this pitch. 

 The followiug data is given to show the manner in which this adjust- 

 ment is made, and to show the effect of such a resonant adjustment in 

 increasing the sensitiveness of the apparatus. 



An organ-pipe Fj 4 giving 705 complete vibrations per second, sup- 

 plied by air from bellows operated by an electric blower and set up in 

 the Constant Temperature Room 2 of the Jefferson Physical Laboratory, 

 served as source of the sound. 



The telephone receiver, having a resistance of 53.8 ohms, and pro- 



2 This room is described in Professor Sabine's paper on " Architectural Acous- 

 tics, Part I, Reverberation," published in the American Architect, Vol. XLVIII, 

 April-June, 1900, and in Contributions from the Jefferson Physical Laboratory, 

 Vol. IV, 190(5. This room was used in some of the present experiments because the 

 apparatus for producing the sound happened to be in place there. The appara- 

 tus was in use by Professor Sabine, and together with other parts of the appa- 

 ratus, including two of the receiving telephones, was kindly placed by him at my 

 disposal. 



