Recoil Curves as shown by the Hot-Wire Microphone. 285 



meter. It is less sensitive than the galvanometer used with the electro- 

 cardiograph apparatus. 



The galvanometer used for pulse or breathing records is of the same type 

 as the instrument used in the Sound Eanging apparatus. It is less sensitive 

 than the Souttar galvanometer. 



All the records obtained, whether of body movement, apex beat or 

 breathing, are obtained from one of these galvanometers acting in a 

 Wheatstone bridge circuit. 



The timing device is a rotating toothed wheel, kept in rotation by a 

 synchronous motor, and the current which operates it is supplied from a 

 circuit containing a contact opened and closed by a vibrating tuning fork 

 electrically maintained. In this way, the time-wheel can only rotate at a 

 constant speed satisfying the above conditions, and making the record in 

 hundredths and tenths of a second. 



3. The Photographic Apparatus. 



For the production of permanent records, use was made of the automatic 

 developing apparatus employed in gun sound ranging. This consists of a 

 camera with a roll of sensitised paper which can be fed continuously behind 

 a cylindrical lens ; the paper thereafter passing automatically through a 

 developing and fixing bath. The speed of the sensitised paper can be 

 regulated at will, and is capable of being varied from 6 inches per second 

 ■ to inch per second. 



It will be noted from fig. 1, that the diaphragm which receives the 

 body impulses is very highly damped by the rubber ring that supports it. 

 The microphone container is purposely made double, with rubber connection, 

 since the latter will damp out very efficiently any resonance the air in the 

 system might have. 



The vibration galvanometer is heavily shunted so that, although sensitivity 

 is sacrificed, the instrument is practically dead-beat. Fig. 2 shows the record 

 of a make and break of 1 ohm change in the resistance of the microphone. 

 This record also indicates sensitivity, and enables us to standardise all records 

 taken from day to day (Plate 5). 



With a standard microphone grid, working with a given electric current, 

 and with a galvanometer of one definite sensitivity, quantitative comparisons 

 of the various records can be made. The recording of minute air currents 

 by the microphone is a special feature of our method as distinct from that 

 employed by Yandell Henderson, and has been dealt with in a paper by 

 one of us (W. S. T.) and Capt. E. T. Paris, on "The Selective Hot-Wire 

 Microphone " (15). 



