rot y : portional to the logarithm of input current over a range in excess of six decades, Since the units were to be self-cantained and to operate unattended at remote locations they were powered by batteries, Wariations in operating characteristics due to gradual decrease of battery voltage were accounted for by prcviding for periodic self-calibration, This was achieved by injecting known currents into the circuit at four-hour intervals by a cam- operated switch on the clock-driven recording milliameters, The units operated satisfactorily for approximately five days which was the maximum length of time the clock motors operated. The low currents to be measured together with the high values of resistance in the circuit necessitated exclusion of all water vapor, This was achieved by containing the electronics in an air-tight housing pressurized slightly with dry nitrogen gas, ! 5 IONIZATION CHAMBER i The limiting sensitivity of the electrometer, 19712 amperes, together with the requirement that the apparatus possess a sensitivity to garama radiation ot 5 mr/br, dictated that the ionization chamber have a sensitiv~ ity of about 10°? ampere/r/hr. Since nc commercial ionization chambers of this sensitivity were available it was necessary to develop one with the appropriate characteristics. The resulting chamber was a high Pressure, multielectrode Argon chamber with a sensitivity of approximately 2 x 107 amp/r/hr, Figs, 2 and 3 show, respectively, an exploded view of the chamber, and a view of the assembled chamber together with the electrometer housing. 6 PROTECTIVE HOUSING The electrometer, ionization chamber, recording miliiameter and bitteries were contained in rugged aluminum boxes which could be handled by two men and could withstand exposure to any weather which might be encountered at those locations wh:sre no protective shelter was available, Aluminum radiation shields placed over the boxes precluded any "greenhouse" effect from occurring, An undue temperature rige in the boxes could have undesirable effects; a high temperature could create irreversible changes in values of the components in the electrometer leading to instability or undue drift, seriously decreasing the life of the batteries and an increase in the ionization chamber pressure beyond the limit of the protective pressure seal, Figure 4 shows the interior of the housing containing all equipment. Figure 5 is a view of the apparatus completely enclosed, Provision was made to allow operation with the ionization chambers at a location remote from the electrometer. proper and manyof the units on Eniwetok were so employed with the electrometers ingide blockhouses and the chambers inside protective canisters located outside. 7 CALIBRATION Calibration of the ionization chambers waa carried out by comparison