INSTRUMENTATION New t r a n s i s t o r s , m u l t i v i b r a t o r circuits, oscillators, stepping switches, a n d v a c u u m t u b e v o l t m e t e r s hold promise of w i d e spread use in a n a l y t i c a l c h e m i s t r y by Ralph H. Müller THE countless advances in new tubes, OFcircuits, and applications thereof, an attempt is made to describe some selected developments which seem to hold promise of rather widespread application in analytical chemistry. In the past, smooth control of the anode current has required the phase shift method of control. The development of the Plasmatron by RCA laboratories is described by E. O. Johnson of RCA [Electronics, 24, 107 (1951)]. This developmental tube achieves smooth control as an inherent tube property, rather than circuitwise. The Plasmatron is a hotcathode helium-filled diode capable of controlling large currents continuously at low voltages. A small current of 12 ma. yields an anode current of 950 ma. at an applied anode potential of only 6 volts. The dynamic response is high. The gain is a function of frequency, and is constant and equal to the steady or d.c. gain up to 2 or 3 kc. At 10 k c , it is still 8 3 % of maximum and appears to drop off rapidly above that value, to attain about 5 0 % at 20 kc. The plasmatron will undoubtedly attain widespread use in control and measuring circuits. Transistor Theory and Practice Progress in transistor theory and practice is no longer a matter of reference to isolated publications. The November 1952 Proceedings of the Institute of Radio Engineers contains 48 original articles, an editorial, and three general commentaries on the present status of transistor theory, devices, and applications. This is little short of a monograph on this subject. Two of these contributions cannot fail to be of interest to the analytical chemist. K. Lehovec [Proc. Inst. Radio Engrs., 40, 1407 (1952)] deals with new photo-
electric devices utilizing carrier injection. This paper discusses the detection of injected carriers by their absorption and emission due to their recombination with majority carriers. On the basis of this principle, two new components are offered for consideration: a phofomodulator which allows the modulation of a light beam by the variation in absorption because of injected carriers; and the "graded seal junction" which permits the light to be transmitted to and from a P-N junction without substantial absorption in the bulk semiconductor. I t is likely that the phofomodulator combined with a phototransistor may be useful as an amplifier. A "perfect" photocell is the topic of another of these papers [Shive, J. N., Proc. Inst. Radio Engrs.. 40, 1410 (1952)], which deals with the properties of the M-1740 P-N junction photocell. This new phototransistor employs a special P-N junction and has a sensitivity of 30 ma. per lumen for light of 2400° K. color temperature. This corresponds to a quantum yield (electrons per incident photon) of approximately unity in the spectral range from the visible to the long-wave cutoff at 1.8 microns. This enormous intrinsic sensitivity is not to be confused with the performance of photomultiplier tubes, because the latter owe their high gain to internal amplification by a secondary electron emission process. The new photocell element is encapsulated in a 1/t X Vie X Vβ inch plastic housing. Space is not available for a systematic review of transistor theory and practice. However, a thorough review of this subject and its implications is urgent, and the analyst will need such information very soon. Modern techniques in electrodeposition and coulometric analysis require d.c. 33 A
sources of great constancy, and many electronic schemes for this purpose have been described. One of these, by J. K. Taylor [Rev. Sci. Instr., 22, 481 (1951)], furnishes highly regulated low voltages at rather heavy currents. This particular design employs a novel principle. A relaxation oscillator of controllable period feeds pulses of constant amplitude to a power amplifier and rectifier stage. The output voltage is balanced against a refer ence voltage and the resultant error signal is fed back regeneratively to control the pulse repetition rate. Multivibrator
Circuits
Multivibrator circuits have been used for a long time for precise switching opera tions, but usually for milli- or microsecond intervals. Two recent developments have extended their usefulness to a range of seconds or even minutes. In one of these, b y II. L. Armstrong [Electronics, 26, No. 9, 202 (1953)], two gas triodes use an R C circuit to control the firing cycle of each gas tube. A relay is actuated by the firing of one triode and switches itself to the action of the second triode which, upon firing, in its turn releases the relay and starts the cycle of operations over again. Only one set of contacts on the relay is required fcr the cycling operation, so that if a D.P.D.T. relay is employed, the extra pair of contacts can operate external con trol or timing operations. Like the ordi nary multivibrator, this circuit can be ar ranged in a stable, monostable, and bi stable form. A related technique was developed by J. K. Merlis and J. Degelman [Science, 114, 692 (1951)] for an artificial respirator for animal experimentation. This useful device affords an automatic instrument for
ANALYTICAL
34 A
INSTRUMENTATION
MODEL 230
AUTOMATIC FRACTION COLLECTOR
alternately admitting air or other gases to a system and connecting the system to exhaust at rates varying between 5 and 40 cycles per minute. A double-ended piston in the pump is driven by solenoids. The latter are excited alternately by recti fied a.c. which is switched to the solenoids by a relay. The relay is in the anode cir cuit of a 6J5 tube, which is controlled by the output pulses from a controlled multi vibrator. Rate stabilization is obtained by an OD3 voltage regulator (ube. A modification is suggested by the authors, in which two 6L6 power tubes can be controlled by the multivibrator to drive the solenoids directly and thus dispense with the relay.
BOTH TIME AND DROP COUNTING OPERATION FOR THE MOST PRECISE FRACTION CUTTING REQUIREMENTS OF CHROMATOGRAPHIC WORK
• One outstanding feature is the phototube arrangement for drop counting. Drops from the column fall directly into the test tubes without touching anything. There are no intermediate glass arms and funnels to cause mixing, contamination, evaporation, etc. This is especially important when accurate separations are required. Turntable, phototube housing, etc., are made of Stainless Steel.
Price complete for time and drop counting $795.00
-SEND A CARD OR LETTER FOR COMPLETE
INSTRUMENT COMPANY
PACKARD
INFORMATION-
RESEARCH EQUIPMENT AND SERVICE
LA GRANGE, ILLINOIS
P. O. BOX 428
rfutoeUve NO-LEAK Speed 1/aive HELD AT 6 0 0 0 PSI FOR 3 0 DAYS PRESSURE DROP BiGISTIREP T h i s valve was photographed t w i c e . . . 30 days apart. As y o u c a n see, pressure was c o n s t a n t over t h e test period. Dependable performance is t y p i c a l of AUTOCLAVE E N G I N E E R S ·
products. Another i m p o r t a n t AE feature is service. T h e valve a n d all fittings used for t h e test are i n stock i t e m s . A p o i n t t o r e m e m b e r w h e n y o u t h i n k of h i g h pressure equipment. JAN. 10, 1953
For more about SPEED VALVES. WRITE FOR BULLETIN 6152
FEB.
AUTOCLAVE ENGINEERS, INC. ERIE, PENNSYLVANIA EXPORT DEPT.
751 DREXEL
BLDG.
CHEMISTRY
PHILADELPHIA
10,
1953
Further developments in vacuum tube voltmeters are concerned with attempts to increase stability, raise the input imped ance, and lower the input capacitance. The most recent improvement in this direction, described by A. W. Brewer of the Clarendon Laboratory in Oxford [/. Sci. Instr., 30, No. 3, 91 (1953)], utilizes an electrometer tube in conjunc tion with an ordinary pentode or triode, both acting as cathode followers. A volt age gain of 0.90 is attained with input leak age current of the order of 1 0 _ H A. and input capacitance which is a small frac tion of 1 microfarad. By the further addi tion of two pentodes acting as "tail tubes," the input range can be extended to ± 5 0 0 volts, although a power supply of 1200 volts is required for this range. The power dissipated by the second tube is great enough to operate an ordinary voltmeter or recorder. Another logarithmic amplifier extend ing over a range of 1 to 106 has been de scribed by M. Briere, A. Rogozinski, and J. Weill [J. phy.i. radium, 12, 144 (1951)]. It has been known for a long time that a logarithmic relation exists between the current through a diode and its anode voltage when the latter is negative. This phenomenon has been reinvestigated in the region of very weak currents. A cir cuit is described using a multigrid elec trometer tube in diode connection, and operated at subnominal rating. It is fed from various sources such as ionization chambers or phototubes. The logarithmic portion of the current-voltage curve ex tends over five decades. The lower limit depends upon the tube, but in some cases currents as small as 10™14 A. follow this relationship. The importance of logarith mic elements or circuit is much greater than one might suspect, because they enter into several of the prevailing meth ods for high-speed multiplication in which logarithmic terms are fed into sum ming or difference amplifiers. Oscillators
β, ΡΑ.
The intentional introduction of jitter or oscillations into measuring instruments
36 A
ANALYTICAL
CHEMISTRY
INSTRUMENTATION
When Accuracy . · • S ^ Dependability Are important . ^
y
Mate tkeéA vefiAaiiL·
LABORATORY AIDS FREEZE-DRY APPARATUS FREEZE-DRY APPARATUS A complete, compact a n d m o b i l e freeze-dry l a b o r a tory for various methods of freeze-drying. Equal drying rate o n a l l openings; large drying capacity; large-capacity 2-stage v a c u u m system. M a n y other advantages based on t h e work of outstanding authorities are given in B U L L E T I N N-2240.
AUTOMATIC PIPETTING MACHINE Delivers accurately measured q u a n t i t i e s of liquid» into ampules, vials, etc., u n d e r aseptic conditions, in volumes quickly a d j u s t a b l e f r o m 0.025 t o 60 m l - , a t constant speeds f r o m 10 t o 150 strokes per m i n u t e . Average error is less t h a n 1 % . No hanging drop a t delivery t i p . Syringe, valve assembly a n d t u b i n g easily detached as a u n i t for s t e r i l i z a t i o n . Selected v o l u m e is securely m a i n t a i n e d w h i l e in o p e r a t i o n . Compact—occupies only 8 % χ 6 i n . of bench space. B U L L E T I N N-52-122-123.
AUTOMATIC P I P E T T I N G MACHINE
HIGH-SPEED ANGLE CENTRIFUGE A reliable, modern high-speed centrifuge w i t h unique balancing mechanism t h a t makes for greater speed and safety. R.F.C. developed: 25,000 g a t f u l l speed of 14,000 r . p . m . T u b e angle, 32 degrees. Easily sterilized. F u l l y - a d j u s t a b l e speed c o n t r o l . C a n be used in refrigerators. Self-contained u n i t construc tion—easily carried f r o m r o o m t o r o o m . B U L L E T I N N-52-134.
HIGH-SPEED ANGLE CENTRIFUGE
is an ancient principle first treated by Gouy. It has been used rather frequently in recent years as a means of reducing stiction errors in meters and recorders. A very thorough treatment of the subject by R. L. Ives [Electronics, 25, 161 (1952)] is particularly useful because it is con cerned with a discussion of long-term relia bility of such devices. Relay oscillators with a frequency of 5 to 15 cycles are so successful that they are being incorpo rated into all modern field meteorological instruments. However, for fixed station operation, their life is too short. A good telephone-type relay is dependable for 10,000,000 operations. At 10 cycles per second on 24-hour operation, this is used up in about 11.6 days A line-operated keep-alive circuit employing a 10-cycle multivibrator will yield 40 days of con tinuous service with ordinary tubes. When red tubes (10,000 hours) are used, monthly checking is advisable and tube replacement is indicated every 13 months. This study has shown that the keep-alive frequency injected into the measuring circuit should be between 5 and 15 cycles per second and that the wave form should be nonsinusoidal but symmetrical. Fre quencies of less than 5 cycles per second require excessive amplitude to be effective and insert sawteeth into the record. Fre quencies of much more than 15 cycles per second interfere with ink flow in pens of several designs, converting the recorder into a very efficient ink thrower. Fre quencies in excess of 400 cycles per second make continuous excavations in the chart.
CONSTANT TEMPERATURE BATHS A wide variety for every laboratory need is described in Catalog 52. T h e popular dual-purpose h e a t e d refrigerated bath (illustrated) can be used also for circulating liquids t h r o u g h external apparatus. By a t t a c h i n g a new A m i n c o m e c h a n i s m for shaking flasks, etc., t h i s bath can be used for m i c r o and macro metabolic i n c u b a t i o n . Described in B U L L E T I N Ν-2239.
Stepping
INCUBATORS, OVENS & STERILIZERS Available in a wide variety of t i m e - t e s t e d , reliable types for every laboratory need. Ovens include forced a n d gravity convection, explosion-resisting, and paraffin embedding types. Sterilizers include small electrically-heated table models t o large autoclaves heated by electricity, gas, or s t e a m . Incubators include forced-convection (refrigeratedheated), gravity convection, water-jacketed and a n h y d r i c types. C A T A L O G N-52.
Other Aminco Laboratory
CONSTANT TEMPERATURE BATH
Instruments
Electrophoresis Apparatus W a r b u r g Apparatus Metabolic S h a k e r - I n c u b a t o r Light-scattering Microphotometers P h o t o m u l t i p l i e r Microphotometers F l a m e Photometers Mechanical Dialyzers Optical Absorption Cells C o l e m a n Spectrophotometers, Fluorometers, Nephelometers, Colorimeters, and p H Meters
INCUBATOR
Thermoregulators Relays I m m e r s i o n Heaters P u m p s & Stirrers A M E R I C A N INSTRUMENT C O M P A N Y , I N C . Silver Spring, Maryland ·
In Metropolitan Washington, D. C
Switches
There would seem to be little possibility for further improvement in stepping switches. These have attained a high degree of development in the form of the multiple bank selector switch commonly used in automatic telephone exchanges. However, new designs have been forth coming from several sources. One of these is the new Roto-Stepper manufac tured by the G. M. Giannini Co., Pasa dena, Calif. This device, which weighs less than 1 pound, is capable of unlimited clockwise or counterclockwise rotation. It is solenoid-operated with a normal increment of 2°. I t operates at 24 volts, 0.6 ampere maximum, and can be ordered in various combinations with or without shaft output. Some of these are: shaft output with potentiometer output, shaft output with potentiometer output and homing segments, and the same plus in ternal autocycling. With a minimum pulse length of 0.05 second, the maximum stepping rate is 720 per minute. This can be raised to 30 per second, depending on torque. The torque output varies with the pulse rate, but is approximately 12 inch-ounces at 12 pulses per second.
