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upon the phase of the a.c. output of the amplifier; if this signal leads the line a.c. in phase, the rotor will turn in one direction, if it lags the line, the direction of rotation is re versed. The rotor shaft is me chanically linked to the slidewire contactor so that rotation of the shaft drives this contactor along the slidewire. The mechanical linkage is so arranged that the di rection of drive of the sliding con tactor is always in the sense that will reduce the amplifier output sig nal. That is, if the contactor hap pens to be above the balance point, the potentiometer unbalance signal will be of such a phase with respect to the line a.c. as to turn the motor in the direction that will move the contactor down. If the contactor is below the balance point, the un balance signal will be opposite in phase to the previous case, and the motor drives the contactor upward, again toward the balance point.
Thus, the deviation of the poten tiometer from balance produces a signal that sets in motion a chain of events the consequence of which is to move the slidewire contactor in a direction that reduces that signal. The contactor therefore continually seeks the balance point, for that is the only position in which the motor is not energized. The basic principle of the servomechanism may be generalized in the schematic form shown in Fig ure 6. The signal to be measured is compared with a reference stand ard, and the difference is presented as an error signal to an amplifier, the output of which activates a sig nal generator to produce a correc tion signal that is applied to the source in such phase as to cancel the error signal. The automatic, continuous-bal ancing capability that the servomechanism provides has made pos(Continued on page 37 A)
GUARANTEED SENSITIVITY 10 ΙΟ std. cc/sec MS-9 leak test stations are used to pressure, vacuum and inside out test hermetically sealed units. The helium mass spectrometer permits sealed objects to be certified leak proof at a sensitivity of 10_1° std. cc/sec. V E E C O manufactures a complete line of high vacuum e q u i p m e n t . . . Components, Leak Detectors, Evapo rators, Systems.. accepted as the quality line for over a decade. Automatically or manually operated. VEECO'S
For MS-9 Brochure or Complete Catalog write Dept.**
Veeco VACUUM-ELECTRONICS CORP. Terminal Drive
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Plainview, L. I. N.Y.
HIGH VACUUM & LEAK DETECTION EQUIPMENT
Figure 5. Circle No. 163 on Readers' Service Card 32 A
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ANALYTICAL CHEMISTRY
Schematic of a self-balancing potentiometer
