Sec. 9.11] 



MASS SPECTROGRAPHS 



295 



quency can be made very small only when the ion current is changed very 

 slowly. If the mass spectrum is scanned as it is in nearly all automatic 

 recording mass spectrometers, A/ must be sufficiently wide to allow the 

 galvanometer or recording device to follow faithfully the rapid increase and 

 decrease in ion current as each mass peak sweeps past the collecting electrode; 

 otherwise resolution is lost because adjacent mass peaks overlap, or alter- 

 natively, time and labor are sacrificed in waiting for the signal voltage to 

 decay to the base-line level. For the same reason it is also important to 

 reduce to a minimum the shunting capacitance of the input to the electrometer 

 tube by making the lead from the collecting electrode as short as possible 



COLLECTING 

 ELECTRODE 



25-30 VOLTS 



Fig. 71. Balanced-bridge electrometer circuit. Resistance i?l is usually of the order of 

 10 10 ohms. If an FP-54 electrometer tube is used, the filament current is about 90 ma and 

 R2 is chosen to give a filament bias of about — 4 volts. The other circuit constants are 

 chosen to give the required grid and plate voltages and adjusted to give zero galvanometer 

 current for zero input signal. The total resistance R3 + i?4 + R5 across the galvanometer 

 should equal the critical damping resistance for the galvanometer. If an inverse feedback 

 d-c amplifier is coupled to the output, R\ is connected to the feedback return lead instead 

 of to ground. Except for the galvanometer and the divider R5, the entire electrometer 

 circuit should be carefully shielded and shock mounted. (See also reference 22.) 



and by using shields of large diameter, thus making A/ sufficiently wide to 

 pass the audio-frequency Fourier components of a slow pulse. 



Considerable care must be exercised in mounting and shielding electrometer 

 tubes since their high sensitivity also makes them highly microphonic and 

 extremely sensitive to stray electric and magnetic fields. Drift, transients, 

 and microphonics are best avoided by carefully shock mounting the tube 

 in a rigid brass or copper can which is then evacuated to prevent sound trans- 

 mission and changes in the surface resistance of both the grid resistor and 

 the surface between the tube terminals, mainly by the removal of water 

 vapor. 



The conventional electrometer circuit is a balanced-bridge network such 

 as that shown in Fig. 71. For the FP-54 the power supply is 25 to 30 volts. 



