428 BELL SYSTEM TECHNICAL JOURNAL 



may be controlled. With any of these magnifiers the suspended coil 

 forms a mechanical oscillating system which is of great assistance in 

 correcting the distortion of the cable, and allows signals to be shaped 

 properly with the aid of a simple network of inductance, capacity and 

 resistance. The inertias of the suspended coil and of the controlled 

 devices make these magnifiers insensitive to high frequencies, and 

 while this has some advantages in discriminating against high frequency 

 disturbances, it also sets a rather definite limit to the speed at which 

 they may be used. In order to utilize them as efficient signal shaping 

 devices the natural frequency must be not far from one and one half 

 times the nominal signaling frequency.^ On this account and because 

 their sensitivity decreases roughly as the square of the natural fre- 

 quency to which they are adjusted, the moving coil magnifiers are 

 rarely operated at signaling speeds of more than fifteen cycles per 

 second. As they are easily damaged by relatively small overloads it 

 is not safe to keep them in circuit when the approach of a thunder 

 storm to a cable terminal makes the reception of induced surges in 

 the cable likely. This sometimes results in keeping a cable out of 

 operation for several hours, although the surges would only occasionally 

 cause the loss of a letter if the magnifiers were not subject to damage 

 by overloading. 



A vacuum tube amplifier is free from many of the disadvantages of 

 the mechanical amplifiers. It contains no delicate parts which require 

 skilled manipulation, and once adjusted it maintains its adjustment 

 indefinitely. There is no inherent limitation to the speed at which 

 it may be operated; this being determined only by the requirement 

 that the signal be sufficiently stronger than the interference. There 

 is no practical limit to the amount of power which may be controlled 

 and at the same time it is easy to limit this power and insure that 

 momentary overloading shall not damage the amplifier or the asso- 

 ciated apparatus. A multi-stage vacuum tube amplifier possesses still 

 another important property in that there is practically no reaction 

 between its various stages at telegraph frequencies. For this reason 

 a number of interstage shaping networks may be used, and it will be 

 found that the adjustment of one network is entirely without influence 

 on the effects of the others. 



History of Development of Vacuum Tube Amplifiers in 

 Bell Telephone Laboratories 



The signal shaping amplifier now in use is the outgrowth of studies 

 of the applications of vacuum tubes begun in the laboratories of the 

 '' JVIilnor, A. L R. E., February 1922. 



