REPORT provides t h e power for operation of t h e r e a d - o u t device. T h i s is il lustrated symbolically in Figure 8,B, which p o r t r a y s the t y p e of cir cuit design used in several com mercial direct-reading p H meters [Keegan, J. J., M a t s u y a m a , G.,
THESE ARE TWO IDENTICAL IR MOISTURE TESTERS...
A N A L . C H E M . 33,1292 ( 1 9 6 1 ) ] .
Chopper Amplifier. T h e desir able features of t h e RC-coupled a.c. amplifier can be employed in t h e amplification of a d.c. signal if t h e latter is converted, or "chopped" into an alternating voltage. T h e design principle of t h e very widely used chopper-type amplifiers is (A) conversion of t h e p r i m a r y d.c. signal into a . c , (B) amplification of t h e a . c , a n d (C) rectification, or de modulation, of t h e a.c. back to d.c. Figure 9 shows t h e circuit dia g r a m of a commercial chopper a m p lifier used as a p H meter. I t has, in addition to the features described above, t h e provision of feedback stabilization by returning t h e de modulated signal coming from t h e "electronic converter" in opposition to t h e input signal. T h e read-out is t a k e n from t h e feedback loop. Operational Amplifiers. One of the most recent developments in commercial amplifier instrumenta tion is t h e availability of a series of feedback-stabilized, d.c. amplifiers which a r e of modular—i.e., "build ing block"—construction, and are designed for t h e performance of v a r ious analog computation operations. These are called operational ampli fiers, a n d t h e symbolic representa tions of t h e principal types are given in Figure 10. These amplifier mod ules are extremely useful in t h e con struction of experimental a p p a r a t u s , and h a v e been exploited in a n u m ber of t h e electroanalytical studies cited in T a b l e I I .
ONE IS A BETTER BUY! B e c a u s e i t ' s in s t o c k a t y o u r n e a r b y Will w a r e h o u s e —like a l m o s t e v e r y Will i t e m . A n d i t ' s on its w a y in a b o u t 24 h o u r s — s h i p p e d p r e - p a i d , r e a d y - t o - u s e . B e t t e r y e t , y o u ' r e p r o t e c t e d b y Will's exclusive double guarantee . . . t h e m a n u f a c t u r e r ' s w a r r a n t y b a c k e d u p b y a n unconditional g u a r a n t e e of satisfac t i o n b y Will . . . a n d y o u ' r e a l w a y s w e l c o m e b a c k t o u s e t h e i n d u s t r y ' s finest service facilities. Y o u choose from a l m o s t 1000 different b r a n d s , over 20,000 different i t e m s , w h e n y o u b u y from Will. Y o u r Will r e p r e s e n t a t i v e h a s a c o m p l e t e t e c h n i c a l reference file on p r a c t i c a l l y e v e r y one of t h e m . W e like t o t h i n k y o u c a n ' t m a k e a b e t t e r b u y anywhere.
NEW! MOO RE-MIL FORD IR M O I S T U R E A N A L Y Z E R Easy, accurate readings regardless of sample density or weight. No preliminary weigh-in—simply spread thin layer of sample on t h e p a n and t h e balance automatically gives you the moisture content read t o t h e nearest 0 . 1 % . Extensive testing shows amazing repeatability of results. W h e n drying time is set a n d heater positioned, operation is automatic. Analyzer shuts itself off—answer remains until you change it. N o possibility of reabsorption or careless technique giving incorrect answers. I n stock a t your nearest Will warehouse. Write for technical literature; free demonstration on request. N o obligation.
BIBLIOGRAPHY Bair, E. J., "Introduction to Chemical Instrumentation," McGraw-Hill, New York, 1962. "Basic Electronics," Navpers 10087, U. S. Government Printing Office, Washing ton, D. C , 1955. Donaldson, P. Ε. Κ., "Electronic Ap paratus for Biological Research," Butterworths Scientific Publications, London, 1958. "Radar Electronic Fundamentals," Navships 900,016, U. S. Government Print ing Office, Washington, D. C., 1944. Stacy, R. W., "Biological and Medical Electronics," McGraw-Hill, New York, 1960.
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