1579 
be overcome by means of careful shielding. It would then be possible to 
take advantage of the increased effective sensitivity afforded by lower 
input capacity ,and one could probably adapt the instrument to measure 
sensitivities as low as Nera coul. per psi. 
Further modifications and improvements. 
4.1 The diagram of Figure 19 shows the present stage of development of 
the microcoulometere Various modifications and improvements suggest 
themselves for future versions of the device: (a) Use RCA long life 
(red base) 6SJ7 and 6SN7 tubes,(b) The 6J7 screen balance controls affect 
the sensitivity slightly. These variables should be removed and all 
balancing done by means of the 6SN7 grid control. (c¢) Obtain the 6SN7 
plate voltage by putting two VR 75's in series across the 150V regulated 
supply, and use a 75V plate voltage instead of the 90V battery. (d) 
Install 6E5 or 6U5 shadow tube to permit ready balancing of voltage 
across R1 R, before connecting DV ampe meter. 
4.2 Fairly elaborate precautions must be taken in order to maintain 
an effective resistance of 50 to 100 thousand megohms at the input. 
The entire input stage should be carefully dried and then dipped or 
cast in polystyrene to minimize the effects of surface moisture films. 
Guard rings should also be installed at the input and connected to a 
suitable point in the circuit so as to rise and fall with the input 
voltagee Grid bias of the VX-41A mst be carefully adjusted for minimum 
grid current. 
Use of a Speedomax or Brown recorder as a detecting device. 
= 103 
