Electrometer Tube

a receiving tube used in electronic and electrical measuring instruments to amplify and measure small currents (down to 10^–14 ampere) in circuits with a very high electrical resistance. It may be in the form of a triode (single or double), tetrode, or pentode and usually has a directly heated or indirectly heated oxide-coated cathode.

The principal feature of an electrometer tube is its high input resistance, which is determined by the requirement that the currents on the control grid be low when the grid is at a negative potential. The occurrence of grid current in an electrometer tube is associated with a finite resistance value for the electrical insulation of the grid (the resistance to grid leakage current); with ionization of the residual gases in the tube’s envelope; with thermionic emission from the grid; and with photoemission from the grid’s surface resulting from external illumination, thermal radiation from the hot cathode, and the soft X rays produced by the deceleration of electrons at the plate.

Grid current can be reduced to 10^–15 ampere or lower by applying various technical design measures, the most important of which include reducing the cathode temperature to 750°–800°K, reducing the plate voltage to values lower than the ionization potential of the residual gases (usually down to 10–12 volts), and reducing the dimensions of the control grid and providing it with effective electrical insulation. The grid current is directly dependent on all these factors, except the last. However, it is difficult to achieve low grid currents with satisfactory values of basic parameters (such as the transconductance of the grid characteristic and the amplification factor), primarily because of the photo emission resulting from soft X rays. Thus, with a grid current of 10^–15 ampere, the transconductance of the grid characteristic usually does not exceed 100–120 microamperes per volt and the amplification factor is 1.5; with modified electrometer tubes operating at a grid current of approximately 5 × 10^–11 ampere the parameters are 1 milliampere per volt and 25–30, respectively. The range of current values measured (the ratio of the limiting values) for an electrometer tube is usually about 1:100; for logarithmic modified electrometer tubes (with a characteristic that provides an output signal proportional to the logarithm of the input current), it can reach 1:10⁸.