International projectionist (Jan-Dec 1946)

Some Historic Firsts: The Vacuum-Tube Voltmeter TECHNICAL STAF_r, BELL TELEPHONE LABORATORIES Bl 1915. instruments for measuring d.c. voltage were very satisfactory, and were available in a wide variety of forms. Instruments for measuring a.c. voltage at frequencies used in the power field were also available, although perhaps not quite so satisfactory as the d.c. instruments. Measurements of voltage at high frequencies, however, were much more difficult to make. Hot-wire or electrostatic types of instruments were generally required, and these lacked the simplicity and practical precision of the electrodynamic meters except under controlled laboratory conditions. Careful calibration was required before dependable readings could be obtained. Patent Application Sept. 1915 A disadvantage of practically all types of commercial voltage-measuring devices was that they drew appreciable current from the source being measured. Since most of the high-frequency volt FIGURE 1 age measurements were made on circuits of high impedance, even the small current taken by the measuring instrument disturbed the circuit and affected the accuracy of the reading. At that time. R. A. Heising was actively engaged in radiotelephone developments, and was badly in need of an instrument that would easily and accurately measure voltages at the radio frequencies. The high-vacuum electronic tube had recently been developed by Arnold and had been employed for transcontinental wire telephony. Heising's familiarity with the characteristics of this tube enabled him to perceive that it could be adapted to the measurement of voltage, and in Sepember, 1915. he applied for a patent on a thermionic voltmeter. Patent No. 1.232.919 covering such a device was issued in 1917. and reissued in 1922. Since that time, the vacuum-tube voltmeter has become probably the most widely employed instrument in the radio field. With the development of carrier transmission, and the widespread application of higher frequencies to wire communication, the field of the vacuum-tube voltmeter was further broadened, and today it is indispensable in all electrical communication work. When the grid of a three-element vacuum tube is negative, no appreciable current will flow in a circuit connected between the grid and the filament. Expressed in another way, the impedance across the grid and cathode under these conditions is extremely high — infinitely high in the ideal case. Moreover, when the grid is sufficiently negative, no current will flow in the plate circuit either; and with any value of grid voltage, the current that flows in the plate circuit is unidirectional, and thus may be measured with a simple d.c. meter. These are the characteristics that Heising took advantage of in inventing the vacuumtube voltmeter. Actual Measuring Procedure With the circuit shown in Fig. 1. E is the source of voltage to be measured, and p is a potentiometer with which any desired percentage of the battery voltage may be applied to the input circuit. The CLAYTON BALL-BEARING EVEN TENSION TAKE-UPS For all projectors and sound equipments All take-ups wind film on 2, 4 and 5 inch hub reels. Silent Chain Drives THE CLAYTON REWINDER For perfect rewinding on 2000-foot reels. CLAYTON PRODUCTS CO. 31-45 Tibbett Avenue New York 63, N. Y. ;•■ uLcmcter may be given a calibrated scale so that the applied voltage may be read directly. To make a measurement of voltage, the switch at the left is first moved to the lower position, and the potentiometer is turned until the meter M just shows zero current. The switch is then moved to the upper position. If E is an alternating voltage, the current that flows in the plate circuit is related to the average value of the positive half-cycles. The potentiometer is now moved until STRONG REFLECTORS INTERNATIONAL PROJECTIONIST • September, 1946 33