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The Keithley Model 6105 Electrometer is a versatile instrument which measures wide ranges of dc voltages, currents, resistances and charges. It is a highly improved form of conventional dc vacuum tube voltmeters that uses an electrometer tube input to provide greater than 10¹⁴ ohm input resistance, The Model 610B has all the capabilities of conventional VTVMs, but it can also make many more measurements without loading circuits. The Electrometer has 11 voltage ranges from 0,001 volt full scale to 100 volts, 28 current ranges from 10-¹⁴ ampere full scale to 0.3 ampere, 25 linear resistance ranges from 100 ohms full scale to 10¹⁴ ohms, and'15 coulomb ranges from 10-¹² coulomb full scale to 10-⁵ coulomb. The Model 610B input stage uses a matched pair of electrometer tubes. The output stage uses a vacuum tube; otherwise,solid-state devices are used for all Amplifier stages and the Power supply. This increases reliability and stability and allows greater regulation of the Power supply. a. Voltmeter accuracy is ±1% of full scale, exclusive of noise and drift. b. Zero drift of the Model 610B Is 200 microvolts per hour maximum averaged over any 24-hour period after warm-up. During the 2-hour warm-up, zero drift is no more than 2 millivolts after the first hour. c. Two Amplifier outputs are available. A switch on the rear panel allows either ±3 volts or ±1 milliampere for full-scale meter deflection. The current output is variable ±5% with 1.4-kilohm recorders. The unity-gain Amplifier output is equal to the input voltage within 50 ppm or 100 microvolts, exclusive of zero drift. d. Current measurements can be made by one of two methods, the normal method in which the current is determined by measuring the voltage drop across a resistor shunting the input, or the fast method in which negative feedback is applied through the shunt resistor. The latter method reduces the input drop and greatly increases the response speed on the low-current ranges. DIFFERENCES BETWEEN THE MODELS 610B AND 610BR, a. The Model 610BR Is the rack version of the cabinet configuration, Model 610B, The circuits, specifications, electrical parts and operating procedures for the two models are the same. Besides the outside dimensions, the main difference between them is the Model 610BR's COARSE ZERO Control is on the front panel, not on the rear panel. b. The instructions in the Manual are for both models, although only the Model 610B is mentioned. Any differences are identified for the correct model.