ACCELERATION EFFECTS ON ELECTRON TUBES 1213 



systems at amplitudes that are measurable with the help of a microscope. 

 The peak acceleration in gravitational units for sinusoidal motion is 

 given by: 



g = .102 (c.p.s.)^ X amplitude (inches) 



At higher frequencies, this method of calibration becomes increasingly 

 difficult due to the decrease in obtainable amplitudes. A calibration 

 method for a wide frequency range is described in Reference 3. In gen- 

 eral, the sensitivity of an accelerometer is a function of the active ma- 

 terial, its size, and the weight M employed. The useful frequency range 

 increases with decreasing sensitivity. For a given design, a compromise 

 therefore has to be made between these quantities and the over-all per- 

 missible weight of the finished unit. To illustrate the approximate rela- 

 tions of weight, size and sensitivity, units have been constructed in the 

 shape of miniature tubes weighing only 33 per cent more than their 

 prototypes, with sensitivities in the order of 0.005V rms/g and a useful 

 frequency range of 3,000 cycles. 



Since these accelerometers are calibrated for rectilinear motion, the 

 results obtained in measuring equipment vibrations or testing machines 

 must be carefully interpreted. In many instances the disturbing forces 

 impart a rocking motion to the units so that the position of the active 

 elements in the housings will influence the acceleration magnitudes that 

 are registered. 



Associated Instrumentation 



Since the voltage output of self generating accelerometers is small, 

 electronic amplifiers have to be employed to bring the signal to desired 

 levels. Fig. 7 illustrates a typical arrangement of the necessary equip- 

 ment. A cathode ray oscilloscope is shown as the visual indicating means, 

 although other recording instruments can be employed, depending on 

 the nature of the disturbances to be measured and the type of record 

 desired. The prime requirements of the equipment are: 



(a) phase distortion must be low, so that the disturbance pattern is 

 correctly presented. 



(b) its transient and frequency response must be adequate for the 

 frequencies to be recorded. 



In the circuit shown in Fig. 7, the signal is fed into a cathode follower 

 stage having a high input impedance in order to obtain good sensitivity 

 and frequency response. The use of a cathode follower also offers more 

 flexibility in the proper matching of the high impedance pick-ups to suit- 

 able low pass filters or to the following amplifying stage. The generated 



