The Model 150A Microvolt-Ammeter is a stable, versatile instrument for measuring extremely low level DC signals. It functions as a voltmeter from one microvolt to one volt full scale, and as an ammeter from one milliampere to one hundred micro-microamperes full scale. It also operates as a DC amplifier with pains up to ten million for driving recorders. The very low noise level of the Model 15>0A, together with its long term stability make it ideal for many measurements requiring extreme power sensitivity. Typical applications include measuring the output from strain gages, thermopiles, thermocouples, bolometers, phototubes, ionization chambers, scintillation counters, and barrier layer cells. Other applications are found in cell studies, measurement of electrochemical potentials, electrolytic corrosion studies, molecular weight analysis and Hall effect studies. In addition to its use as a direct indicator of minute potentials and currents, the Model 150A may also be used as a null detector in Wheatstone or Mueller bridges, or with an external voltage source as a meg-megohmmeter. An important feature of the instrument is zero suppression up to 100 times full scale, in place of the usual more limited meter zero. This permits measurements of stmall signals in the presence of large thermal EMF's or other masking DC signals.