INSTRUMENTATION A hot-cathode helium-filled diode that can control large currents continuously at low voltages may achieve smooth control as an inherent tube property by R. H. Müller /~i KTD-controlled gas discharge tubes, ^-* such as the Thyratron, have long enjoyed widespread use in numerous measuring and control devices. The chemist has encountered these tubes chiefly in thermostatic control systems or as components in servo-systems. The thyratron, to use the term in a generic sense, has had certain characteristics which were at once both advantages and disadvantages. Aside from certain definite changes in electrode parameters, a thyratron is essentially a triode or tetrode, in which the envelope is filled with an inert gas at an optimum pressure, either mercury vapor or one of the noble gases. Under normal operating conditions, conduction between cathode and anode is prevented by a suitable negative bias on the control grid. When a sufficiently positive potential, with respect to the cathode, is applied to the control grid, conduction ensues and the resultant anode current sets in, and is limited only by the applied anode potential and the load resistance. Like any arc device, the thyratron has a negative resistance characteristic, and the maximum current is entirely dependent upon circuit-limiting conditions. Contrary to "hard tube" behavior, the grid of a thyratron cannot regain control of the anode current by making it more negative, at least for reasonable values of grid-cathode potential. This apparent disadvantage is put to good use in certain "lock-in" characteristics. For example, in many instances, it is highly desirable to utilize this characteristic in detecting a condition, or state of affairs, where a continuous or persistent indication of a transient event will prevail until an observer takes cognizance of it, applies a corrective measure, and once more resets the thyratron. All this can be done automatically, if the anode voltage is applied periodically—i.e., if a.c. is applied to the anode. Under these con-
ditions, the thyratron will regain control once every half-cycle, because whenever its anode is negative, the arc discharge must cease, regardless of control-grid potential. A distinctive property of the thyratron has been its relatively high current-carrying capacity, low potential drop across it, and the ease with which it can be switched from the nonconducting to the conducting state. I t was essentially an "all or nothing" device from which the term "thyratron" was derived. I t was not until Langmuir, in this country, and Toulon in France, developed the phase-shift technique, that it was possible to achieve a more or less smooth variation of anode current from zero to maximum. These techniques used reactive circuit elements to achieve a displacement in time between the grid and anode voltages and thus fix the fraction of the positive cycle during which the tube could conduct. This has been a widely employed means of getting smooth output control of these power devices and of overcoming an inherent limitation of the device. Fundamentally, the large current-carrying capacity of a thyratron depends upon the neutralization of space-charge around the electron-emitting cathodes, by the positive ions of the gas discharge. Plasmatron
Recently, the R.C.A. laboratories at Princeton have produced a developmental tube called the Plasmatron which gives good promise of achieving smooth control as an inherent tube property, rather than circuitwise. As described by E. O. Johnson of R.C.A. [Electronics, 24, 107 (1951)], the Plasmatron is a hot-cathode helium-filled diode capable of controlling large currents continuously at low voltages. A small control current sets on an auxiliary discharge that provides the ionization to neutralize space charge. The main cathode of the Plasmatron is oval and is surrounded by a U-shaped 23 A
anode. Facing the open end of the U is the auxiliary cathode, which is completely surrounded by a cylinder that acts as a constricting grid. A narrow aperture in the latter permits the auxiliary discharge to pass into the main part of the tube, where it controls the plasma density and hence the main anode current. The increase in the plasma density arises from the fact that the constriction in the discharge path raises the voltage drop across the auxiliary discharge. This imparts more energy to the discharge electrons and increases their ability to ionize the gas. Some of the positive ions and electrons recombine in the interelectrode spaces of the tube, but at the 1-mm. pressure of helium used, these losses are negligible. The steady-state density of charged particles in the plasma depends upon equilibrium between generation and loss rates and is very nearly proportional to the magnitude of the auxiliary or controlling current. The presence of the plasma, of controllable density, between the main electrodes permits the passage of large currents between the main cathode and anode, even at anode potentials of a few volts. As a matter of fact, anode potentials higher than the ionization potential of the gas cannot be used because continuous ionization and conduction would be maintained independent of the auxiliary current and the control feature would be lost. In practice, an ordinary vacuum tube such as 6J5 is placed in series with the auxiliary current circuit. A variation of the grid-cathode potential of the vacuum tube will then vaiy the auxiliary current and thus control the output current. A typical tube characteristic for the Plasmatron shows the anode current as a function of the auxiliary or controlling current. This is almost linear and is slightly concave to the control current axis. In a typical case a control current of 12 ma. yields an anode
24 A
ANALYTICAL
CHEMISTRY
AVAILABLE INSTRUMENTATION
9
Reasons Why
KERN LK-30
Micro-electrophoresis Apparatus The K e r n LK 3 0 a l o n e features all nine of these outstanding advantages.1 / / / / / /
Accurate Rapid Easy to operate Portable Compactly constructed Versatile
The only electrophoresis appara tus for quantitative analysis ot MICRO quantities (0.1 cc. oi serum). The only apparatus that makes possible a complete analysis within lYx hours from start to finish. The only instrument that gives a picture of distribution of refractive index over the entire horizontal cross-section of the cell. The LK 30 uses interferometry*. *Ref. — L. Θ. Longsworth, Anal. Chem., p. 344. Feb. 1951.
G e f the full défaits the Kern LK 30 by for complete literature
abouf writing today!
KERN COMPANY 5 Beekman St., New York 38, Ν. Υ.
current of 950 ma., at an applied anode potential of only 6 volts. The average slope of this curve corresponds to a current amplification of about 90 to 1. If an attempt is made to exceed a cer tain output, the retarding field at the cathode disappears and the output sat urates at a limit set by temperaturelimited emission of the cathode. The dynamic response of the Plasmatron is encouragingly high. The gain as a function of frequency is con stant and equal to the steady or d.c. value of gain up to 2 or 3 kilocycles. At 10 kc. it is still 8 3 % of maximum and appears to drop off rapidly above that value, to attain about 50% at 20 kc. This behavior is characteristic of all gas tubes and is determined by the rate of diffusion of the changed particles to the electrode surfaces. The time constant of plasma decay varies directly as the gas pressure, directly as the square root of the mass of the gas atoms, and as the square of the geometric di mensions of the plasma region. Experimental forms of the tube have been used for direct speaker drive in audio amplifiers, in motor control cir cuits, in pulsing circuits, and in other electronic applications. I t promises much in applications requiring low fre quency, low impedance performance whose continuous variation of relatively large currents is required. The an alyst may expect to see the Plasmatrôn employed in many devices for control or measurement. No extensive data are as yet available concerning life or reproducibility, but from the experience gained on older types, usually operating under much higher voltages, the prospects seem very favorable for dependable service. Patents on Electron
Tubes
Some years ago we had the opportunity to examine a comprehensive collection of patents dealing with electron tubes. It was astonishing to note the advanced thinking and skilled experimentation which these depositions revealed. Very little of this information has ever appeared in the more formal scientific literature, and perhaps no more than 5% of these devices have ever reached commercial production. In the scientific sense, this is unfortunate, though understandable. At the time, we gained the impression that (Continued on page 3β A)
REPRINTS OF Reviews
and Symposia from ACS Publications
MateOaU 1st 2nd 3rd 4th
oj
REVIEW. . . #R-4 . . . REVIEW . . . #R-14 . . . REVIEW . . . #R-20 . . . REVIEW . . . #R-30 . . .
$0.50 $0.75 $0.50 $0.75
1st REVIEW . . . #R-13 . . . $0.50 2nd REVIEW . . . #R-19 . . . $0.50 3rd REVIEW . . . #R-28 . . . $0.50
3rd REVIEW . . . #R-10 . . . $0.50 4th REVIEW. . . #R-15 . . . $0.50 5th REVIEW . . . #R-22 . . . $0.50
AttaLftiaal
GUemUfruf
1st REVIEW . . . #R-16 . . . $1.50 2nd REVIEW . . . #R-24 . . . $1.50 Items R-16 and R-24 . . . $2.50
Absorption and Extraction . . . #R-26. . . $0.75 Adsorption #R-27 . . . $0.50 Atmospheric Pollution #R-21 . . . $0.75 Sulfur #R-31. . . $0.75 Titanium #R-23 . . . $0.50 Key to year of publication: 1947 — R-l through R-9. 1948 — R-10 " R-15. 1949 —R-16 " R-21. 1950 — R-22
ORDER FROM: Special Publications Dept. American Chemical Society 1155—16th S t r e e t , N.W. W a s h i n g t o n 6, D.C.
ANALYTICAL
26 A
The
CHEMISTRY
INSTRUMENTATION
ADVANCES IN CHEMISTRY SERIES Bring»
you
A 48-PAPER SYMPOSIUM OF 273 PAGES ON
AGRICULTURAL CONTROL CHEMICALS AT $2.50 PER COPY 7 PAPERS IN 73 PAGES ON . . .
ANALYTICAL METHODS IN THE FOOD INDUSTRY AT $1.50 PER COPY
24 PAPERS IN 170 PAGES
. < : >
ο**
2£I SEND ORDERS
SEARCHING THE CHEMICAL LITERATURE AT $2.00 PER COPY
TO:
SPECIAL PUBLICATIONS DEPARTMENT AMERICAN CHEMICAL SOCIETY 1155—16th Street, N.W., Washington 6 , D.C.
almost every conceivable means of elec tron-flow control had been tried and evaluated. The more we think about it, the more profitable it seems to exhume some of these devices and re-examine them in the light of present-day prob lems. The lack of wide scale in dustrial application accounted for much of the situation, but it is altogether likely that many measure ment and control problems are being studied today at great expenditure of time and effort, for which some of these forgotten devices would supply an immediate answer. To the best of our knowledge, the movable anode vacuum tube is about 20 years old. Only recently it has been revived and put to use in measuring surface finish, as a sensitive micrometer and as the pickup element in record players. At least as many years ago, English investi gators modified a cathode ray tube in order to deflect the beam into two or more cups, collectors, or "Faraday cages" and used the device for high speed switching purposes. In their hands, it was developed into an anticollision device for aircraft. The economic excuse for this state of affairs is good enough, but the infor mation, for the benefit of students and investigators, is largely hidden and in in accessible form. Our own ingrained and chronic prejudices incline us to the view that this is a natural consequence of the general attitude toward instrumenta tion. In the past, it has seemed per fectly logical to assume that the problem comes first. After it is defined, one then seeks a means for its solution. With the growing interest in instrumen tation for its own sake, one seeks every conceivable mode of measurement, ex amines every obscure phenomenon, and extracts from it those possibilities which will permit a new mode of measurement or control. Recent ex perience has shown that for every dis covery or development of this sort, a dozen or more applications'are at once suggested. Recent developments in crystal physics have led to the varistor, thermistor, transistor, and the phototransistor, and the practical develop ment of these devices has tapped the best resources of chemistry and metal lurgy as well as physics. There is more than a remote possibility that, in some applications, these devices will render the vacuum tube obsolete.
