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BELL SYSTEM TECHNICAL JOURNAL 



requirements as to wave shape, volume of current, etc. One of the 

 earliest forms of oscillators known as the "substation howler" was 

 made by coupling the receiver and transmitter of a subscriber's set 

 together mechanically and taking off the alternating current gener- 

 ated by means of an induction coil in the howling circuit. This type 

 of oscillator, which was subject to large variations in volume and 

 produced a very poor wa\e form, has now been replaced b\- other and 

 improved types. 



Oscillators now in use in the field can be divided into three general 

 classes, those employing vibrators, those employing motor generator 

 equipment and those employing vacuum tubes. The principles of 

 these oscillators are briefly described below by considering one com- 

 mercial type in each class: 



(1) Oscillators Employing Vibrators, l^'is,'. 2(1 shows the circuit 

 arrangement of an oscillator of this t\pe which is designed fur pro- 

 ducing a single frequency alternating current. 



H>-»d2<> — >^ 



Fig. 29 



The current generating element consists of a metal reed R, resting 

 against the diaphragm associated with the carbon button C. The 

 receiver spools M, are so arranged that when they are energized by 

 the battery an attractive force is exerted upon the reed which draws 

 it away from the carbon button. This action decreases the pressure 

 in the carbon button with a corresponding decrease in the current 

 from the batter\-, which in turn decreases the attractive force of the 

 receiver so that the pressure of the reed against the carbon button is 

 again increased. This cycle of change in current and pressure is 

 repeated at the natural fre(|uency of vibration of the reed so long as 

 dirt'ct current llow s front the batterN'. The alternating currents set 

 11]) n this way arc passed tlirough a circuit resonant at the natural 

 l)eriod of \ibr.ition of the reed, llH-ri'by gixing a current of good \\a\e 

 form. 



This (larticular form of vibrator oscillator is iisi-d principalK' in 

 transmission testing work where portable "ear balanc-e" methods 



