26 A
ANALYTICAL
INSTRUMENTATION
Klett.... Conductivity Bridge
furnace, in a choice of atmospheres relative to oxygen content. Automatic records can be taken over 10-hour periods and afford a continuous indication of the oxida tion process. In this installation and similar to other previous methods, the range of automatic compensation is ordinarily limited to the range of the chain block. When this range is exceeded at either the upper or lower limit, it is necessary to add or subtract 100-mg. weights. This can also be done automatically by standard weightlifting mechanisms which are actuated by limit switches. T h e alternative choice of using a heavier chain involves some difficulties in retaining adequate precision. I t is interesting t o note the present status of our means for measuring the three most fundamental quantities—mass, length, and time. While the ac curacy and precision in all three cases are comparable, there is ample evidence to show t h a t length and time and their derivable consequences such as displacement and frequency can be measured in numerous ways and with high speed. Of the three, the one which is of greatest interest to the chemist has received the least attention. Monograph on Vacuum Tube
The bridge, earthing circuit, oscillator, and amplifier are all contained in one m e t a l cabinet.
The balance point is indicated by
a "magic-eye" and by phones w h i c h give a sensitivity of 0.01% or better.
The coils
of t h e decades are adjusted to ± 0.1% of their n o m i n a l resistance.
The range of
resistances is from 0 to 11,111 o h m s , a s h u n t i n g resistor is used when higher re sistances are measured.
Conductivity Cells are available.
Klett
CHEMISTRY
Manufacturing Co.
179 EAST 87TH S T R E E T , NEW YORK, Ν. Υ.
Amplifiers
Comments and recommendations in this column for the M . I . T . Radiation Laboratory series of monographs will not subside until the series is completed. Once more it seems compelling to refer analysts to Volume 18 on vacuum tube amplifiers, edited by G. E. Valley, Jr., and H. W'allman. Chapter I I on direct-coupled ampli fiers is written b y J. W. G r a y and contains information available in no other source. This is particularly true in the compilation of vacuum t u b e characteristics a t low currents, some of which are extremely useful and are not available in ordinary tabulations of tube be havior. We single out this chapter particularly for its importance for the instruments of the analyst, wherein the direct-coupled amplifier is often the only reasonable approach. T h e other excellent chapters cover almost every other aspect of modern amplifier theory and practice. One difficult}' attending the use of these monographs has been their great degree of interdependence, so t h a t the discussion of any one topic is never complete unless one has related volumes at hand. Perhaps more serious has been the inadequate literature references and at times it becomes difficult to decide whether a circuit or technique is brand-ηβλν or 15 years old. Although the editor-in-chief has explained the difficulties and in some cases impossibilities of giving adequate references and acknowledgments, it seems t h a t one must accept this situation and regard the series as a priceless and indis pensable collection of modern techniques.
