86 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



bridge diagonal versus negative pressure applied to the outside of the 

 membrane) is linear within about 2 per cent. The system is tempera- 

 ture sensitive, but the influence of the temperature upon the trans- 

 ducer output can be nearly compensated by a thermistor circuit. 



A similar construction is described by W. Ramberg, J. Research Natl. Bur. 

 Standards, 37, 391 (1946), for use as an accelerometer. The acceleration causes 

 the movement of two plates within the tubes. See also R. Gunn, Rev. Sei. 

 Instr., 11, 204 (1940), and J. Appl. Mechanics, 7 (1940), and also U.S. Patent 

 2,155,420, Feb. 27, 1940, and J. Rothstein, U.S. Patent 2,389,935, Xov. 27, 

 1945. 



Higher sensitivities can be obtained with a triode system, as shown 

 in Fig. (1-2)67 (Mechano-electronic Transducer, RCA Tube 5734). 

 The cathode and grid assembly are held in a fixed position within a 

 vacuum-tight envelope, the anode is supported by a rod which ex- 

 tends through the center of a thin metal diaphragm sealed to the tube 

 envelope. An angular displacement of this rod leads to a variation of 

 the plate current. The transfer characteristic is linear within about 

 2 per cent. The maximum permissible displacement of the rod is 

 ±0.5° about the zero position. For this displacement, a torque of 

 13.3 g-cm is required. The moment of inertia of the moving system 

 is 3.4 rag-cm 2 . The frequency response is limited by the mechanical 

 resonance of the part of the plate shaft within the tube, which is about 

 12,000 cps. The tube is generally operated with the grid connected 

 to the cathode and with a plate supply voltage of 300 volts applied 

 through a 75,000-ohm load resistor. A maximum displacement 

 of the anode rod by 0.5° results in a variation of the output by 

 ±20 volts. Thermal drift and some low-frequency instability at a 

 signal level of 1 m V, as well as variations of the output by 1 per cent 

 of the maximum signal for a variation of the heater voltage by 1 per 

 cent have been observed by Talbot. 1 



c. Ionization Transducer. A glass tube filled with gas at a pressure 

 of about 10 mm Hg and containing two electrodes is brought into an 

 electric high-frequency field between the plates P x and P 2 of a capaci- 

 tor, as shown in Fig. (1-2) 68a. If the field is sufficiently high, a glow 

 discharge will arise in the tube. The two electrodes A and B act as 

 probes in the discharge; their potential is determined by the space 

 potential of the plasma surrounding each electrode and by the rf 

 potential induced by their capacitive coupling to the plates P 1 and 

 P 2 . In the symmetry position the net charges of both electrodes are 

 equal, so that their potential difference is zero. Outside of the sym- 

 metry position, the charges are different for each electrode and give 



1 S. A. Talbot et al., Rev. Sci. Instr., 22, 233 (1951). 



