NEW TRANSMISSION MEASURING SYSTEMS 5 



of the amplifier is a minimum. Moving the potentiometer arm to 

 the lowest step gives less feedback and the amplifier net gain is greater. 



The principle of stabilization is as follows: If a constant potential 

 is applied to the input terminals of the amplifier a constant potential 

 will also be applied to the grid of the first tube. As long as the tubes 

 or the rectifier do not change in characteristics the output of the 

 rectifier will likewise be constant. Now if through any cause, such 

 as a change in tube characteristics or in the rectifier, the gain of the 

 amplifier-rectifier should increase, the output will increase and the 

 neutralizing voltage fed back into the input circuit will also increase. 

 This will reduce the voltage on the grid so that the net output voltage 

 of the amplifier-rectifier will not be changed noticeably. Conversely, 

 if a change in the tubes or the rectifier should cause a reduction in 

 gain, the voltage fed back into the input circuit will decrease, there 

 will be less feedback voltage and the output voltage will be substan- 

 tially the same as before. The reverse feedback amplifier-rectifier 

 is so stable that once it is adjusted to have the proper characteristics 

 it will remain constant for long periods. 



The meters used with this amplifier-rectifier have a range of 15 db. 

 This is less than the required measuring range so that it is necessary 

 to increase it by changing the amplifier gain in steps of 10 db. The 

 reverse feedback amplifier lends itself readily to this as the variation 

 of a single resistance in the feedback circuit is sufficient and no ex- 

 pensive balanced attenuators are required. In Fig. 2 this is done by 

 potentiometer P. In practice this resistance is controlled by relays 

 which form a part of the amplifier, these relays in turn being remotely 

 controlled by keys, jacks or dials at various points in the office. 



The db meters can be located where desired without reference to 

 the location of the amplifier-rectifier. They may also be placed in 

 lantern slide projectors which throw a greatly enlarged meter scale 

 on a screen so that it can be read from distances up to 50 feet or more. 

 The new arrangements are extremely flexible, one set of equipment 

 supplying measuring facilities for several different points in an office 

 where previously several sets would be necessary. Where the use of 

 the equipment is intermittent more than one meter may be used with 

 a single amplifier-rectifier. The meters from which the results are 

 read may be of the conventional indicating type which can be mounted 

 on keyshelfs or on vertical panels, they may be of the projector type 

 or they may be of the recording type which records on paper the 

 characteristics which are being measured. All meters are interchange- 

 able, being similar electrically. 



These new instrumentalities have removed many of the limitations 



