The Keithley Model 410 Micro-microammeter is a line operated vacuum tube electrometer designed and constructed especially for measuring small currents. Full scale ranges are from 10⁻³ to 3 x 10⁻¹³ ampere, The features include full-scale voltage drop at the lnptit of less than five millivolts, zero drift of less than 2% of full scale in eight hours, good accuracy and calibration stability, and simplicity of operation. It also has an output which will drive a O-l or 0-5 milliampere recorder as well as the numerous potentiometer - rebalance recorders; one output terminal is at ground, making it convenient to connect cathode ray oscilloscopes or pen-driving amplifiers, similar to the Brush and Sanborn equipment. The major panel controls are the range switch (amperes fullscale) and the zero. Minor controls are the Zero Check, used to short circuit the input and in setting the zero, Meter Polarity Tar providing up-scale readings for currents flowing in either direction, and an ON-OFF power switch. The mater dial is illuminated, and these bulbs serve as the pilot light.