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Gas-Chrom Solid S upfront fan,
CHROMATOGRAPHY SUPPORT • More than 10 different mesh sizes. • Regular, acid w a s h e d , acid and alcoholic base washed, and silane treated grades.
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S T A T I O N A R Y PHASES • Polyesters, silicones, polyglycols and many others.
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LABORATORIES, INC. Dept. AC-2 140 N . B a r n a r d St. S t a t e C o l l e g e , P a . U . S . A.
PACKINGS • A n y of our stationary phases on a variety of supports.
S6lfi*He*tt faom Sfoc6
Circle No. 11 an Readers' Service Card
TUBES AND COORS AD-99 CRUCIBLES HIGH PURITY, IMPERVIOUS, RECRYSTALLIZED ALUMINA TUBES AND LABORATORY WARE -Coors AD-99 is an impervious, high purity alumina ceramic with exceptionally high refractory properties. It is an ideal material for thin-walled crucibles, thermocouple insulators, protection tubes. • High mechanical strength. • Maximum operating temperatures up to 1900°C, if fully supported. • Inert, even at high temperatures. Coors AD-99 tubes and laboratory ware are made in standard sizes and forms, or in custom forms for special requirements. Write, today, for new catalog showing all standard tube and laboratory items.
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Area Code 3C3-CR9-4533 ' Golden, Colorado
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ANALYTICAL CHEMISTRY
from the positive side of the power supply to its negative side) to the grid of the second stage, the ar rangement is called a direct-coupled (or d-c) amplifier. If the coupling resistor is replaced by a capacitor, the circuit is t h a t of a resistancecapacitance coupled amplifier (the resistance referred to in the desig nation is p a r t of the voltage divider network ; it is the dropping resistor connected between the grid and the cathode, or ground). If the plate load resistor in the first stage is replaced by an inductor, and the first-stage plate is coupled to the second-stage grid through a capaci tor, the circuit is t h a t of an im pedance-coupled amplifier. If the plate load of the first stage is a transformer primary winding, and the grid and cathode of the second stage arc connected across the sec ondary winding of this transformer, any variations occurring in the plate potential of the first stage are transmitted to the grid of the second stage through transformer-coupling. If a d.c. signal is to be directly amplified in a multistage circuit, one must use a direct-coupled am plifier. If the input signal is a . c , any one of the four types of ampli fier m a y be used. The choice is made on the basis of the frequency or range of frequencies of the input signal, and the degree of amplifica tion sought. The RC-coupled am plifier requires a fairly low applied voltage for appreciable gain, and loses its amplification both at low and at high frequencies. I t gives uniform frequency response in the ΑΓ range; it is characterized by minimum distortion of the signal, but only low amplification per stage. Impedance-coupling results in higher gain in the high frequency range, b u t the degree of amplifica tion is not uniform with frequency. Transformer-coupling can give high gains, but the frequency response is poor, and the size and weight of the amplifier tend to be larger t h a n for the other types. Classification scheme G, based upon the design principle embodied in the over-all circuit, is in many respects the most useful and in formative from the point of view of the chemist. The significance of the various terms employed in this
