The Keithley Model 600A Electrometer is an ultra-high impedance voltmeter with full-scale ranges of 0.01, 0.03, 0.10, 0.30, 1.0, 3.0 and 10 volts. The maximum input resistance is greater than 10¹⁴ ohms in addition, the input resistance may be varied in decade steps from one ohm to 10¹¹ ohms by means of the shunt resistors built into the instrument. Thus, the 600A is not only appropriate for measurement in high-impedance circuits, but also can be used where a high input impedance would merely introduce unwanted pickup. The 600A may be used as a direct-reading ammeter from 3 amperes to 10-¹³ ampere full scale. This lU-decade range is covered in overlapping 3x and 10x scales. Two current measuring methods are available to the user of the 600A. Normally, current is determined by measuring the voltage drop across a resistor shunted from input to ground. Alternately, on the 10-⁷ to 10-¹³ ampere ranges, negative feedback can be applied to the input of the voltmeter through the current measuring resistor. This largely eliminates the input drop and increases measuring speed, particularly on the more sensitive ranges. The 600A measures 10K to 10¹² ohms full scale with a two-terminal input. With a guarded input, its range is extended to 10¹³ ohms. Unlike conventional ohmmeters, information is presented on the linear scales used for current and voltage readings. As a dc preamplifier, the 600A has gains of 0.1, 0.3, 1, 3, 10, 30 and 100. The continuing stability of the gain is assured by a feedback factor in excess of 100 on any range. The output is minus 1 volt, open circuit, with an internal resistance of 17K. Thus, the output may be used for driving oscilloscopes or pen recorder amplifiers, or the output may be shunted down to provide 10- or 50-mv outputs for servo-rebalance recorders.