An Adaptation of the Electron Tube to the String Galvanometer.

In coupling the string galvanometer to the output of an electron tube, various devices have been used to avoid placing the string directly in the output circuit. 1 , 2 This has been done to avoid damage to the string because of the high plate voltages used. However, the voltage amplification factor (μ) which is one of the most important constants in amplification equations, is a function of the geometry of the tube.

approximately 3 where p = distance from grid to plate, a = distance between adjacent wires of grid, r = radius of grid wires. Practically μ is found to vary but little within a wide range of plate voltage. Therefore, there is only slight loss of amplification involved in operating at low plate potentials, and there is considerable gain in safety of manipulation. Furthermore, in operating at low plate potentials and normal filament temperatures, one is working on that part of the characteristic of a tube where the plate to filament current is limited by space charge. This tends to reduce fluctuations in the tube circuit which are due to the so called “shot effect.” 4

When low plate potentials are used, the galvanometer may be placed directly in the output, and zero position of the string maintained by applying to the string terminals a potential equal and opposite to the P. D. set up by the passage of the plate current.

The circuit adopted is shown in Fig. 1. It consists of one stage of voltage amplification and one stage of power amplification. The resistances used are practically pure resistance and when the string is placed directly in the output circuit the amplifier is aperiodic.