AUDIO FREQUENCY ATMUSPIIERICS 



499 



600 ohms to facilitate insertion of attenuators and filters. The 

 maximum gain for the three amplifier units was 200 db, attenuators 

 and filters being used at all times to control the output intensity. 

 The amplifier was designed to minimize noise, inherent in such 

 apparatus, and to be highly stable throughout long periods of prac- 

 tically continuous operation. 



In addition to several high-pass and low-pass filters, 17 narrow 

 band filters designed to cover in small steps the range from 150 to 

 3800 c.p.s. were available. A filter switching panel was used to 

 facilitate observations of various frequency ranges in rapid succession. 



The output was arranged to supply various recording and indicating 

 devices. R.m.s. measurements were made by means of a thermocouple 

 with a long period direct reading and recording meter. A device 

 employing three-element gas-filled tubes was used to measure peak 

 voltages. A magnetic recorder was employed in securing a few sound 

 records of atmospherics. Oscillograms which are shown in this 

 article were subsequently prepared from these records. The Ireland 

 amplifier with some of its associated apparatus is shown in Fig. 1. 



Fig. 1 — Amplifier and associated apparatus used at Frenchport, Ireland. 



(1) First amplifier unit 



(2) 1st Attenuator 



(3) 2nd Amplifier unit 



(4) 2nd attenuator 



(5) 3rd amplifier unit 



(6) Recorder 



(7) Band pass filter 



